Since he's no longer on your roster, I'm guessing that he would have been a 5th yr SR (SR*) and not just a SR; a player's progression is FR, SO, JR, SR, 5th year SR.
All SRs expect to start and play about 60% of the snaps. Not all players where this isn't met leave (they will lose WE, though), my guess is that his HC scouting report would have indicated that he wasn't much of a team player/played only for himself. It doesn't happen often, but depending on the position, it can really suck.