There are two things that account for what happens next.

First: Crowley's arms feel like they're made of spaghetti, or perhaps a few lengths of wet string. He has to prop himself up in stages - erratically, and jerkily.

Second: amongst other reasons, he hasn't nearly the presence of mind to furl his wings away.

Thus, Aziraphael - who reluctantly pushed himself not far away enough - finds himself assaulted in very short order by a faceful of black feathers.