Later in the day I tried again, this time with the lead around his neck. Same thing.
Now, since it happened so suddenly, I'm wondering if its from pain? He has a sunburnt nose that I've been trying to keep sunscreen on (before he decided not to be caught). Also, the other day he freaked out when tied and broke his halter, so I'm wondering if he got hurt then, or at least developed a bad association with the halter?