The B/N switch somehow moved to the B (bulb mode)
This. This has happened to me more times than I care to admit. And sometimes it's just because I'm trying out a different camera and I think I remember which was bulb and which was snapshot (because not all the oldest cameras have any indication on which is which) but I remember it wrong and voilà, shaky photos!
he he i see.
survived this Holga test. After years of using Holga this happened to me first time :-)