Surely something must have motivated him to go to Real Betis. Sometimes there are factors we don't know about in the decisions of players and coaches. For example, Mourinho refused to go, despite being overpaid, to play in the Arab League. Maybe his wife doesn't like the climate or something else in that part of the world.
Yeah, something must have made him to go there, like you said different factors plays a key role in moves like this. Last season when he was struggling at Manchester United, Real Betis came in and offered a deal for him, he came into the club and impress and the club showed interest in wanting to buy him. It's just natural that he honours it and again, maybe another bigger club didn't come for his services that's another factor too.