Why did Vishnu in Vamanavathar push Bali into the netherworld?

Question

For what fault or great sin did Vishnu punish Mahabali by pushing him to Pathala?
Mahabali was known to be a pious person, even direct paternal grandson his great devotee Prahlad , a king kind to his people whom he ruled very well.

As per the declaration - "Yada Yada hi Dharmasya Glanir Bhavathi Bharatha, Abhyuthanamadharmasya Tadatmanam SrijanyaAham'."

The God takes self-birth Avatars to destroy evil sinners and re-establish dharmic order. But Mahabali Emperor had no bad track record, even was directly blessed by Vamana himself before deportation to Pathala and so on, then what was the cause or provocation?

Answer 1

But the Mahabali Emporor had no bad track record

This does not seems to be the case.

As per Bhagwat Purana, below situation is mentioned:

Bali Mahārāja, after offering obeisances to Prahlāda, the brāhmaṇas and his spiritual master, Śukrācārya, equipped himself to fight with Indra and went to Indrapurī with his soldiers.Blowing his conchshell, he attacked the outskirts of Indra’s kingdom.

So, basically, he was attacking and trying to acquire the property/wealth of Indra, by force. That does not seems like a good quality.

Also as you mentioned, King Mahabali was a generous man who won the praise of the world.

But the flip side of this is mentioned in Wikipedia:

This praise, from his courtiers and others, led him to think of himself as the greatest person in the world. He believed that he can help any one and can donate whatever they ask. Even though he became benevolent, he became pompous of his activities and forgot that the almighty is above him.

Bali become proud of his possessions(again, not the best of the qualities) as identified and explicitly spelled in S.B.8.21.34

vipralabdho dadāmīti  tvayāhaṁ cāḍhya-māninā tad vyalīka-phalaṁ bhuṅkṣva  nirayaṁ katicit samāḥ

Meaning: Being falsely proud of your possessions, you promised to give Me land, but you could not fulfill your promise. Therefore, because your promise was false, you must live for a few years in hellish life.

After gaining all the strength, he become a threat to universe itself, as mentioned in Bhagwat Purana, S.B.8.15.26:

nainaṁ kaścit kuto vāpi  prativyoḍhum adhīśvaraḥ pibann iva mukhenedaṁ  lihann iva diśo daśa dahann iva diśo dṛgbhiḥ  saṁvartāgnir ivotthitaḥ

Meaning: No one anywhere can counteract this military arrangement of Bali’s. It now appears that Bali is trying to drink up the entire universe with his mouth, lick up the ten directions with his tongue, and raise fire in every direction with his eyes. Indeed, he has arisen like the annihilating fire known as saṁvartaka.

Now, of course, with this kind of situation, As per "Yada yada hi dharmasya glani.." declaration the God takes self-birth Avatars to destroy evil sinners and re-establish dharmic order.

Hence, Vamana Avatar.

Answer 2

Shukracharya recognized Vamana as Lord Vishnu himself.So he tried to dissuade King Bali from fulfilling his promise. Shukracharya was aware that Lord Vamana would cover Bali's entire kingdom (the universe) with two steps, not leaving any space for a third step. Bali rejected his guru's advice because it contradicted with the principle of Dharma.

Bali Maharaja said, "I do not fear hell, poverty, an ocean of distress, falldown from my position, or even death itself as much as I fear cheating a brahmana."

On hearing this, Shukracharya curse that he would suffer in hell for disobeying his Guru's words.

There is also an another reason for King Bali's abandonment form earth.When Vamanadeva had appeared in the womb of Aditi, Prahlada(Grandfather of King Bali) had warned Bali that the Lord would soon defeat the demons and return the heavens to the demigods.

Bali had arrogantly said, "Who is this Vishnu compared to my demon warriors?"

Upon hearing this, Prahlada had him: "How can you speak like this? I curse you to lose your prestigious position and your kingdom and will be banished from earth,for you have insulted my worshipable Lord Vishnu." Infact the story of Mahabali is an example of total surrender and true devotion.

Further Vamana justifies his action of punishing Bali for removing Indra from his position before the expiry of his tenure.(Dialogue between Banasura,son of King Bali and Lord Vamana)