My take, the "conspiracy" against Jesus had been building for at least 3 years,and sources record seven instances of official plotting against him, two efforts at arrest, and three assassination attempts before this time. This intrigue was no spur of the moment idea. What pushed it over the top or help spur on the indictment, the cleansing of the temple.
A formal decision to arrest Jesus had in fact been made several months earlier. The Jewish religious officials were afraid that if Jesus were to continue performing his signs, he would win over the people and the Romans would come in and destroy the Temple and nation.
According to legal custom at that time, a court crier had to announce publicly or post an official "wanted" handbill in the larger towns of Judea about forty days prior to a trial. Small wonder that there was some debate over whether Jesus would dare appear in Jerusalem for the next Passover. This discussion ended abruptly on Palm Sunday.
There were political reasons for dealing with Jesus. There had been a dozen uprisings in Palestine in the previous 100 years, most of them subdued by Roman force. Another Messianic rebellion under Jesus would only shatter the precarious balance of authority, break Rome's patience, and might lead to direct occupation by Roman legions.
Religiously, Jesus was a dangerous item. The people were hailing the Teacher from Galilee as something more than a man, and Jesus was not denying or blunting this blasphemous adulation. Personally, the Pharisees had been bested by Jesus in public debate, being called vipers, whitewashed tombs, and devourers of widow's houses. Humiliated, they would be only too happy to conspire with the scribes, elders, and chief priests.