Okay, I don't have the full story with all the context and background details, but there was some guy who had a vendetta against the Avengers and wanted to take them down by tearing them apart. This was all happening around the same time the UN was debating the Sokovian Accords which would have put the Avengers and any other superhero under the UN, so they'd only be able to act under UN orders. This guy framed Bucky in a terrorist attack on the UN that killed King T'Chaka of Wakanda to flush him out of hiding, then managed to manipulate things so Steve and Bucky would go to Siberia because they thought he was going to dig up some other Winter Soldiers that were still in stasis there. What he actually wanted was for Tony to follow them to arrest Bucky. From what I've put together he made sure Tony found out that Bucky had arranged for his parents to die in a car crash while he was brainwashed. They got into a fight which involved Tony blowing off Bucky's arm before he and Steve got away and Steve went to break the others out of the Raft. So, you know, pretty bad feelings on both sides.