It wasn't his decision would be my experienced bet; they probably offered at least one third to half as much again on his initial base and told him he'd soon be a starter as they refreshed their team/squad once more - he, his advisors and his family would all think this brought him security - and his family most likely thought putting down roots was just as important. But all this will have been compounded by appointing a new manager who has made it known he's not yet convinced he is worth the effort...
His club now has to convince him to stay and fight for a place, but my guess is they've not bothered or have done that poorly - I haven't seen one comment from anyone at City saying they want him to stay (a la RDZ & Bergval for example)...
So, he pushed the club, gets told they have not decided, and he hands in a transfer request; keeping a player who doesn't want to stay, rarely ends well.
But in the transfer market City are known for their arrogance and treating players like shit when it suits them ..