Hey mate, yeah the theory is that Thurston has actually been a negative influence at the club; the roster they have capped off with the best halfback in the world as captain means they should be making the top four regularly, but they haven't succeeded at all since 2005.
The big rumour is that the Cowboys, and Thurston, both want to part ways as they feel they are not good for each-other; the Cowboys supposedly believe they won't achieve anything with Thurston as the leader, and Thurston apparently conversely wants to start fresh.
That's all just speculation though; I've heard it in many nooks and crannies, but it doesn't make it true; I honestly hope he stays at the Cowboys (they are one of my favourite teams) but if both parties believe they will both benefit from parting ways, then I fully support them