Ah! Actually I think I know what could have caused it! Did you by any chance step on a cracking floor tile?
Happened to me - in 1.1.1 - when I brilliantly stuck my hand into an arrow slit in one of the entrance levels.
What, gimme a break! I just had to be sure! :P
Anyway, as soon as I pulled back my bloody stump, my whole party had turned their backs on me. Can't say I blame them, but there you have it.
Really no big deal imo, but thought you might like to know.