Figures this would have to happen on my first day back to bike-commuting. Damnit!
I think this happened due to 1) old frame 2) 130mm rear hub in 126mm frame and 3) the locknuts on the Paul hub are very large/wide so they probably torqued the dropouts in a weird way, in conjunction with (2).
I probably should've respaced the frame when I mounted the new rear wheel. Sigh.