I agree, Yvain said it first, and it doesn’t sound like group selection.
Concerning your group selection comment, that does sound plausible… but being relatively unfamiliar with tribal behavior, I would want to be sure that greedy genes were not spreading between groups before concluding that group selection could actually occur.
The following argument comes from an intro sociology text:
If there are three people competing, all of different strengths, it is worthwhile for the two weakest people to ban together to defeat the strongest person. This takes out the largest threat. (Specific game-theoretic assumptions were not stated.)
Doesn’t this basically explain the phenomenon? If Zug kills Urk, I might be next! So I should ban together with Urk to defeat Zug. Even if Urk doesn’t reward me at all for the help, my chances against Urk are better than my chances against Zug. (Under certain assumptions.)