One other possibility...this did happen to Viohazard once after we'd been away for a month or two over summer, leaving the violin and bow undisturbed in a rather old case...insects had chomped their way through the NEW bowhair (i.e. it wasn't so heavily rosined up at the ends as old bow-hair, so it was tastier, apparently). Nothing apparent when he picked up the bow, but the minute he applied pressure, it all fell out
P.S. Probably wouldn't happen in a cooler, drier climate, but since then, I make sure there is a sachet of insect repellant powder in the case.