Krishna was honored at Yudhisthira's Rajasuya Yagna. This made Shishupala angry. Why was he mad and enraged which made him insult Krishna?
Why was Shishupala enraged at Pandavas when Krishna was honorred during the Rajasuya Yagna?
https://www.wisdomlib.org/hinduism/book/mahabharata/d/doc118386.html The reasons why shishupala got mad and enraged were indirectly given by him during his rant. Following passages describes those reasons.
However, Shishupala could not bear to see that worship offered to Lord Krishna. He spoke up influenced by envy and hatred, "O descendant of the Kuru race, this Krishna does not deserve royal worship as if he were a king. You sons of Pandu are acting like children. Bhishma, the son of Ganga, obviously, possesses little knowledge. He deserves to be censured amongst the honest and wise. How does this one of the Yadu dynasty, who is not even a king, deserve worship before all the exalted kings present? Krishna is not certainly the oldest in age, for here is His father, Vasudeva. If you regard Krishna as a well wisher, there is a greater well wisher present here as King Drupada. When Drona is present, how could you regard Krishna as a greater preceptor? Do you regard Krishna as greater in wisdom than Dvaipayana Vyasa? Amongst the warriors present, the great Bhishma, who can die only at his will, stand supreme. How could Krishna receive worship before him? When Ashvatthama, the knower of every branch of knowledge is present, how could you worship Krishna? When Duryodhana, the bull of the Bharata race, is present how could Krishna be worshipped first? Passing over Kripa, the preceptor of the Bharata princes, why have you worshiped Krishna? When Bhishmaka, Rukmi, Ekalavya, Salya or Karna are present, how could you worship Krishna? This slayer of Madhu is neither a sacrificial priest, nor a preceptor, nor a king. With what understanding have you worshiped Krishna? O chief of the Kurus, you have rejected qualified persons to be first worshiped from motives for gain. If you had planned to worship this Krishna, then why have you brought all these kings here to be insulted? This wretch born of the Vrishni race has caused the slaying of King Jarasandha by unrighteous means.
I fully agree with the respectable persons present here that they are competent enough to select the personality who can be first worshiped, but I cannot agree with the statement of a boy like Sahadeva, who has spoken so highly about Krishna and has recommended that Krishna is fit to accept the first worship in the sacrifice. I can see that in this meeting there are many personalities who have undergone great austerities, who are highly learned and who have performed many penances. By their knowledge and direction, they can deliver many persons who are suffering from the pangs of material existence. There are great rishis here whose knowledge has no bounds, as well as many self- realized persons and brahmanas also; and therefore, I think that any one of them could have been selected for the first worship because they are worshipable even by the great demigods, kings and emperors. I cannot understand how you could have selected this cowherd boy, Krishna, and have left aside all these other great personalities.
"We cannot even ascertain as yet to which caste this Krishna belongs or what His actual occupational duty is. Krishna does not belong to a respectable family. He is so independent that no one knows His principles of religious life. It appears that He is outside the jurisdiction of all religious principles. He always acts independently, not caring for the Vedic injunctions and regulative principles. Therefore He is devoid of all good qualities."
"Under these circumstances, how can He be fit to accept the first worship in the sacrifice? Krishna is so foolish that He has left Mathura, which is inhabited by highly elevated persons following the Vedic culture, and He has taken shelter in the ocean, where there is not even talk of the Vedas. Instead of living openly, He has constructed a fort within the water and is living in an atmosphere where there is no discussion of Vedic knowledge. And whenever He comes out of the fort, He simply harasses the citizens like a dacoit, thief or rogue."
A large part of rant of shishupala indirectly tells us what reasons were for shishupala to get enraged at lord krishna being honoured.