Just my take, but maybe Sunderland humbled him a bit? He got too used to the cozy times at City.
Possibly he's self aware of all this now and wants to build himself up again, and Hibs is a club that doesn't demand too much so he can build. I think he'll do well there.
This is completely an outside look in, for all we know Hibs was the only club interested and he's just taking it because he can.