I think there was more social pressure for him to resign from FSF, and more institutional pressure for him to resign from MIT/CSAIL. The institutional pressure probably wasn't as painful, but was more forceful. I think he'd probably been holding back the pressure he felt to get the FSF thing over with for a while, but hoping for the controversy to die down, but when he was forced (or practically forced) to resign from MIT/CSAIL it became clear he wasn't going to win, and he decided to get it over with. Perhaps he'll be sleeping better tonight.