Board Thread:General Discussion/@comment-4161952-20130331221843/@comment-7683123-20130402124731

Technically, if you were originally a shinigami, you would be a visored when you gain a hollow mask. That means he wasn't an arrancar and did not succumb to the 'turning to dust' effect of aizen's creations after death. Aizen is precautious and perhaps he foresaw Tosen's betrayal after Ichimaru's. That was probably why Tosen's death happened as he died after getting beat up by komamura and hisagi so badly. That's the 'why' answered. The 'what' is trickier since he sliced Harribel after noticing her failure but he didn't even turn around to see Tosen. However, he should've been able to sense his reiatsu fall and thusly killed him.