Yea I think it tends to happen with players 25+ when they think they should play and you just don't play them or transfer them. It's happened to me 4 or 5 times when I sign an older player as a backup and they inevitably get pissy when i don't play them and they're on a long contract.
yah its happened to be me loads if you transfer list a player and dont play him then put him in the reserves and he keeps getting disillutioned with his time at the club.... then they just say i will terminate my contract for nothing or a certain fee if they dont think they will find another club in the mean time.