Are you sure about that? I'm not saying that isn't true but this is the first I've heard of that.
"Best" case scenario: He got passed over by other players and dropped on the depth chart during spring practice & thought he could get more playing time at another school as a graduate transfer.
Worst/more realistic scenario: He thought the D would stink in 2018 so he wanted to play for a school that had a legit chance at a NC or as the OP stated, he saw the writing on the wall in regards to grades/ his status on the team & split.
Either way this sucks. Hayes got better every year he played and even if he wasn't going to be a starter, he was still going to be a very valuable back up on a defense that had/has a chance to be very good in 2018.