I have read that maharishi Bhrigu was choosing which diety to invite to their maha-yagna and he was testing the tri-murties (Shiva,Vishnu, Brahma ). During his test he was disappointed with Shiva and Brahma. He eventually cursed Shiva and Brahma. I wonder how is this even possible. Bhrigu is said to be son of Brahma. A man who was born on earth has such power to put curses on the creator and the destroyer. Or what I read is wrong ?
Maharishi Bhrigu was one of the Manas Putras of Brahma, whom he created right after the emergence of Rudra from his forehead. As mentioned in the Bhagvat Puran:
Then from Lord Brahma's eyebrows appeared a baby, blue-red in color. The baby appeared in eleven forms and began to cry immediately after his arrival. Lord Brahma consoled him! "Don't cry my son, you will be named as Rudra."
Thereafter, Lord Brahma created ten more sons- Marichi, Bhrigu, Kratu, Pulah, Pulatsya, Angira, Atri, Vashishtha, Daksh and Narad. Nine out of them took permission for the creation of their offsprings, but Narad accepted celibacy and resolved to preach Bhagwad bhakti (devotion for God.)
They are the oldest inhabitants of our Universe after the Trinity and the Sanat Kumars so they should not be considered mere human rishis. This is corroborated even in the Mahabharat Shanti Parva which also states that the Brahmanas had the right to punish even the gods:
The Brahmanas are distinguished above the deities and Asuras, since they were created by myself in my indescribable form as Brahma. As I have created the deities and the Asuras and the great Rishis so I have placed the Brahmanas in their respective situations to punish them occasionally. In consequence of his licentious assault on Ahalya, Indra was cursed by Gautama, her husband, through which Indra got a green beard on his face. Through that curse of Kausika Indra lost, also, his own testicles, which loss was afterwards (through the kindness of the other deities) made up by the substitution of the testicles of a ram. When in the sacrifice of king Sarjiati, the great Rishi Chyavana (of Bhrigu clan) became desirous of making the twin Aswins sharers of the sacrificial offerings, Indra objected. Upon Chyavana insisting, Indra sought to hurl his thunderbolt at him. The Rishi paralysed Indra's arms.... When Rudra addressed himself for the destruction of the triple city belonging to the Asuras, the preceptor of the Asuras, viz., Usanas, provoked beyond endurance, tore a matted lock from his own head and hurled it at Rudra. From that matted lock of Usanas sprang many serpents. Those serpents began to bite Rudra, at which his throat became blue.
In fact the human species was created afterwards:
For the continuation of the creation, Lord himself appeared as a female from the left hand and as a male from the right hand of Lord Brahma. Those males and females forms were named Manu and Shatarupa respectively. They were the first in the whole creation to copulate and bring about sexual reproduction. They begot two sons- Priyvrat and Uttanapad, and three daughters- Devhuti, Akuti and Prasuti.
Furthermore, Maharishi Bhrigu is also the father-in-law of Lord Vishnu as his daughter Shri is married to him. The details of his wedding and progeny are also given in the Bhagvat Puran:
Together they (Kardama and Devahuti, the daughter of Manu) enjoyed those luxuries for many years during which nine daughters were born to them. All those daughters symbolised the Navadha Bhakti (nine kinds of devotion) and were married to nine Prajapatis like Marichi etc. Thus kala was married to Marichi, Anusuya to Atri, Shraddha to Angira, Havirbhu to Pulatsya, Gati to Pulah, Shanti to Atharva, Krita to Kratu, Khyati to Bhrigu and Arundhati was married to the sage Vashishtha........ Geat sage Bhrigu and his wife Khyati begot two sons- Dhata and Vidhata, and a daughter- Shri.
In fact even the other rishis of the Bhrigu clan were also so strong that there benediction led Bali to win the Swarg from Indra. As per the Mahabharata again:
The great Rishi Dadhichi of Bhrigu's race is now engaged in performing severe austerities. Go, ye deities, unto him and solicit a boon from him. Do ye so arrange that he may cast off his body. With his bones let a new weapon be created called the Thunderbolt. .... With the Thunder-bolt thus made with the bones of a Brahmana, which was impenetrable by other weapons and irresistible and pervaded by the energy of Vishnu, Indra struck Viswarupa the son of Tashtri. Having slain the son of Tashtri thus, Indra severed his head from the body. From the lifeless body, however, of Viswarupa, when it was pressed, the energy that was still residing in it gave birth to a mighty Asura of the name of Vritra. Vritra became the foe of Indra, but Indra slew him also with the Thunder-bolt.
Similar account of the powers of the Bhrigu clan come from the Bhagvat Puran also:
When Indra saw that Bali had come with full preparation and that his armies had surrounded Amaravati, he went to his teacher Brihaspati and asked about the reason for the demons' sudden uprising. Devguru said, "Indra, blessing of Bhrigu Brahmins is the main reason of the demons uprising. You can't face the demon armies yet, because Kaal himself is in their favor. So you all abandon the heaven and hide somewhere." After the abdication of the heaven by the gods Bali captured it. Then he conquered all the three worlds. With advice of his priests, Bali organized one hundred Ashvmegh Yagyas. Thus by the grace of Shukracharya and other Brahmins of Bhrigu clan, king Bali got the domain of the heaven and began to rule it generously.
The same purana also mentions this testing of the trinity and they mention that Bhrigu was appointed to test them by mutual consensus of other rishis. Probabaly the reason for that was his close relation with all the gods:
Once upon a time, all the sages assembled on the bank of Saraswati river. A dispute errupted among them as to who among the trinity was the greatest. By consensus they appointed sage Bhrigu to test the trinity of Brahma, Vishnu and Mahesh. Bhrigu reached to Brahma, but did not greet him. Brahma got infuriated but since the sage Bhrigu was his own son, He did not curse him.
Then Bhrigu visted Lord Shiva. Lord Shiva proceeded to embrace the sage. But instead of accepting Lord Shiva's welcome, the sage began to abuse Him, saying: "You violate the dignity of Vedas, so I will not meet with you." Angry Lord Shiva raised his trident to kill him, but Bhagawati stopped Him.
At last Bhrigu reached Vaikunth to see Lord Vishnu. Lord was lying with head in the lap of Lakshmi. Bhrigu kicked hard at His chest. But instead of getting angry, Lord got up, bowed his head before the sage and begged pardon: "O Sage, pardon me, I could not welcome you at once, at your arrival. Your feet are soft, I hope they are not hurt. All my sins have been washed by the touch of your feet." Bhrigu felt very pleased by the serious talks of Lord. His heart felt overwhelmed with excess of devotion. He returned to the assembly of the sages and narrated his experience. Since then Lord Vishnu is regarded as the Supreme and giver of peace and fearlessness.
So Maharishi Bhrigu could probably dare to do what he did during the testing of the three gods of the Trinity since he was a powerful rishi who was related to all the three gods of the Trinity - Brahma was his father, Rudra his brother and Vishnu his son-in-law!
It was not only Bhrigu who cursed Brahma and/or Vishnu and/or Shiva, there were others too :
Kashyapa cursed Lord Shiva (discussed here)
Vrinda (Jalandhar's wife) cursed Lord Vishnu (discussed here)
Tulsi (Shankhachuda's wife) cursed Lord Vishnu (discussed here)
There are few other examples too and if you include incarnations in this list then, it's even bigger. So your Question boils down to How people get power to curse even Trimurtis?
जनि आचरुज करहु मन माहीं। सुत तप तें दुर्लभ कछु नाहीं।।
तपबल तें जग सृजइ बिधाता। तपबल बिष्नु भए परित्राता।।
तपबल संभु करहिं संघारा। तप तें अगम न कछु संसारा।।
Marvel not, my son, to hear this; for nothing is too difficult to obtain through penance. By dint of penance Brahma creates the universe; by dint of penance Vishnu assumed the role of it's protector. By dint of penance, again, Shambhu destroys the world; there is nothing in this world which cannot be attained through penance. ~English Translation Source
Similar Chaupayis can be found in Ramcharitmanas - Baal Kaanda - 73 too.
भगवाननुगावाह यातं मा भैष्टमस्तु शम् ।
ब्रह्मतेजः समर्थोऽपि हन्तुं नेच्छे मतं तु मे ॥२९ ॥
The Lord then said to His attendants, Jaya and Vijaya: Depart this place, but fear not. All glories unto you. Though I am capable of nullifying the brāhmaṇas’ curse, I would not do so. On the contrary, it has My approval. ~English translation source
In another story, Brihaspati and Shukracharya also tested tri-murties (Shiva, Vishnu, Brahma).