There have been rumours floating around since January that Mason was on his way out, with Killie suggested as a possible destination back then. His performances in recent months may be because he knows he's heading elsewhere and he can't be bothered giving 100% - especially if that would see his future club relegated!
If Mason could start playing the way he did last season he'd be more than welcome to stay, but as it is he won't really be missed.
Story source: The Scottish Sun