Board Thread:General Discussion/@comment-5398287-20130809180907/@comment-5398287-20130809185922

Ah, that's something I forgot to address.

See, while Ichigo was still around Isshin and Masaki was still alive, Zangetsu remained dormant because of the Gigai Isshin was inhabiting. But, when Masaki died, the suppression ended. However, Zangetsu simply didn't emerge until Ichigo tried to activate his own Shinigami powers, hence the Hollowfication.