I could imagine multiple factors played a part. I'm sure our short term offer was pretty low and I can imagine not playing, but having to be around Fir Park most days, can make you want to move nearer to family and friends again. Plus he would have fallen down the pecking order again and likely thought a fresh start as a first team regular would be better than a bit-part player with the media and fans scrutinising every performance to see if he had regained his prior levels.
Turnbull's interview at the weekend gave an insight into how tough psychologically a long term injury can be - and he's a local with a full local support network.