Imoen erratic behavior

I was just entering the Gnoll stronghold with Khalid, Jaheira, Dorn, Imoen and Minsc . Minsc had just started to seriously whine about rescuing Jaheira.

I entered the stronghold and was just starting to cross the bridge. All of a sudden, Imoen turned around and whaled away at Minsc and killed him off. I can't figure out why. To solve it, I just let Minsc go and picked up Rasaad to finish the quest.

But what was it that caused this behavior? TIA