Well, I think is was a bug bite. :/
He's back to normal behavior, the back of his neck is a little swollen and scabbed over and looks like a bug bite to me. Since cleaning out the box and laying down the thick layer of DE I haven't had any other problems with any of them. Yesterday another one started acting like he got bit too, so I grabbed him up and fluffed out all his neck feathers to dislodge any biting insect that might have been in there. He got a lovely massage out of the whole ordeal. He fell asleep while I was doing it.

Seemed to be fine afterwards.
The box is not OSB, or any kind of pressed board that I can tell, it was a shipping crate in it's former life. Good to know about the glue thing though for future reference. If they started dropping now I would be beyond saddened.
Thank you all for letting me worry out loud here!

I am so glad it wasn't something worse.