Nothing. I read where the poster mentioned Yannick having been with the Vikings, insinuating that he may have had issues with the team. Yannick had no off-the-field issues or locker room problems with the Vikings. The Vikings traded for him, with one year left on his contract, as the Vikings believed they were contenders and wanted to strengthen the defense. They traded him mid-year when they realized they were not making the playoffs. It made no sense to keep Yannick when he would leave at the end of the season and the Vikings would get nothing for him. The Vikes traded him to recoup some of what they gave up.
There are many reasons why players move from team to team.