Yes, it can happen due to a forest fire or another tribe killing them. Although they don't invade each other, dwarves are hostile to elves and could attack them while passing through their village, it's quite unlikely though.
I guess the problem is the message here, because it's not you who killed them. But I don't know how to phrase it in such a way that it works in this circumstance. I guess I could try to detect who it was that killed a tribe.