Thanks for the replies, guys. The wall is finished and painted now - all is well.
It was just such an odd and random thing. I've seen bigger bubbles before, but these were (maybe) about 1/16th of an inch. And very random. 15-20 in one spot, then none over most of the rest of the surface.
This is a rental house that I bought 4 years ago. It had a mold problem that we eradicated. I wondered if maybe there were some lingering mold spores on the wall, but the paint I was mudding over was applied after the mold was taken care of.
Thanks again guys!