Figured it was contaminated pads, from having wheel off for air transit, and maybe getting something on the cardboard stopper I stuff inbetween the pads in transport.
Someone please chime in if I'm missing something, but you only need to put a wedge between the pads of hydraulic brakes to keep them from overextending, causing them not to withdraw into the calipers, necessitating you push them back in with a large screwdriver or specialty tool. Mechanical disc pads, OTOH, can be actuated without a rotor in place without any issues. Or at least I've had no problems with my mechanical discs in the 19 years I've had them on bikes.