25:1 Now three days after Festus had arrived in the province, he went up to Jerusalem from Caesarea. 2 And the chief priests and the principal men of the Jews laid out their case against Paul, and they urged him, 3 asking as a favor against Paul  that he summon him to Jerusalem—because they were planning an ambush to kill him on the way. 4 Festus replied that Paul was being kept at Caesarea and that he himself intended to go there shortly. 5 “So,” said he, “let the men of authority among you go down with me, and if there is anything wrong about the man, let them bring charges against him.”(Acts 25:1-5 (ESV)
Festus was different from Felix in almost every respect except that of the office he now held. Within three days of his arrival in the province, he is in Jerusalem meeting the Sanhedrin. It’s actually quite incredible when one considers all the logistics that must have gone into moving even back then.
The Sanhedrin has an open floor to bring up any concerns and they bring up Paul straightway. They aren’t going to let this go. That they bring him up so soon shows the extent to which they thought he was a threat.
Festus seems to have been briefed concerning Paul himself, which tends to indicate that Felix was very well aware of the political ramifications the imprisonment of Paul entailed. The Sanhedrin is still trying to take what they perceive as justice into their own hands. They want to ambush and kill him on the way to Jerusalem.