Perez and Antraxlotti claim he was scared of battling for his spot with Isco and thus wanted to go while Özil's obnoxious father says they wanted him to leave. Özil says nothing. The truth is probably somewhere in the middle but it is strange that Mesut actually said during a press conference one week before the transfer that he will surely stay.
My guess is that Madrid will regret that. Now they lack the player type Özil is. Furthermore one of the problems of the last season was that he had no competition at his position. So everyone was glad when Isco was signed. Now with Özil gone they have the very same problem plus not knowing if Isco will be as injury-free as Özil was. Which I doubt because he plays with a higher intensity and risk than Mesut. They should have kept him, scrapped Bale and put the 50 million into the Castilla.