It started after Arenado signed the extension with Rockies, he was told or at least thought that they were going to be building around him, acquiring more players and push the Dodgers for the West title. Reality set in when Arenado realized that instead of competing and getting better, the Rockies were going the opposite way, regretting the signing and trying to find a way to get of it.I don't know know all the details, but he has definitely alienated Arenado in recent years.
All last season Arenado feuded with the owner and GM, basically saying if the team is not willing to spend money, he will opt out after the 2021 season. The Rockies reached the point where they were willing to let him opt out, but were afraid that we would stay and they would be on the hook for the rest of 6 years at 199 million.
They are send 50 million to St. Louis to get out of the contract, even if Arenado would opt out after the season, the Cards will still be 15 ahead, 50 - 35 owned to Arenado this season.