User blog comment:Salubri/The Thousand-Year Blood War arc Plot Points/@comment-24234990-20131130191339/@comment-11051098-20131130194046

I don't know. It seems to be something like that.

He went to the world of the living without requesting permission, and then went back a 2nd time despite being warned not to do it again. And he transferred his Shinigami-powers into Masaki's soul to stabilize it after White's attack. Both were illegal actions, and the latter could possibly have been punished with execution. It's basically the same as Rukia did when she transferred her powers to Ichigo.

I suppose he didn't take the chance to go back to SS and risk getting captured and punished.