As I read Vachana Bharatha:
While one could understand the animosity Sisupala had towards Krishna who carried away the girl he was to marry, the extent of anger that Bhishma, an elder, generated just because he suggested Krishna for the Agrapooja was again revealing. Sisupala calls Bhishma weak minded!
When Sisupala storms out of the Sabha, Yudhisthira follows to placate him and Bheeshma seeing this tells his grandson not to waste his time. 'Yudhisthira! Don't try to convince him. He is not worth it. Krishna is superior to all the others here, I do not see anyone who has the strength to defeat him. The reason why we offered him pooja first. Also many wise elders have praised Krishna's qualities. We did not honor him because he was our relative. We considered his fame, bravery and his victories before choosing him. Usually brahmins who are wise and elderly or a khastriya who is the most powerful are worthy of this honor. Krishna has both the qualities. In addition who can match his generosity, ability, knowledge, courage, politeness, fame and intelligence? Sisupala has complained without knowing all this. No king who is knowledgeable would object to this!
Bheeshma must have spoken within the earshot of Sisupala, who retorts: 'As an elder are you not ashamed to talk like this, trying to scare the other kings? It is like the blind leading the blind. How can you praise an arrogant and foolish person like Keshava. Your tongue should split into hundred strands for having spoken thus. How could a wise elder praise a cowherd? He then belittles all the known achievements of Krishna. He calls the Govardhana hill Krishna lifted to protect his people an anthill! He brands him a sinner for having killed a woman, Pootana. Concludes that the pooja which was performed was just useless. He then goes on to verbally attack Bheeshma ridiculing all he did and considered praiseworthy even by gods.
'Bheeshma you speak dharma but do not follow it yourself. The fact you brought home Amba who had given her heart to another was not dharma. Your brother who rejected her acted in the right manner. Your brahmacharya is useless. You have not served any elder to learn dharma from him. A man who has no son is just wasting his time doing yagna and yaga and other vratas. You, a childless old man, who teaches false dharma will be killed the same away as the old swan which stole eggs from other swans for food, was killed.'
Sisupala continues his tirade against Krishna and gets even angrier as Bheeshma narrates the story about Sisupala's birth and the heavenly predictions. He threatens Bheeshma: 'You are alive only because these kings here have tolerated you so far. I have not seen anyone like you who is foolish enough to take on the whole world!'. Bheeshma retorts contemptuously: 'Oh! Is it because they have let me be, I am alive? I do not consider these kings even equal to a blade of grass!'
While this remark amused some of the kings, some others were upset. They all shout: 'This old man is too much! Let us not forgive this insult. Let us all together kill him like cattle or put him on fire!' Bhesshma is not cowed down by this threat. He counters: 'Let us not waste time in idle talk. There is no end to it. First hear my suggestion, then you can decide to either kill me or burn me. We have already performed the pooja and have honored Krishna. If anyone is willing to die, challenge him for a fight.' Sisupala accepts the challenge and fights Krishna and meets his end.