Harry got paralyzed again when he specifically broke one of Mab's laws (regarding how the Fae deal with prisoners, I believe). The Mantle just completely left him, since he was rejecting acting like the Fae, or breaking Mab's will, or whatever.

This book, he was just really tired and injured a bunch. I think it's the difference between driving with a near-empty tank, and removing the gas tank all together.