So many groups have noticed this effect that it seems like it must have been something about the game itself but it's hard to pin down what.
My recollection of Traveller massacres was something like this:
1. Some irresponsible player has their character fire on the first NPC they see.
2. A segment of the party think this is a criminal action, and fire on the PC in protection of the innocent NPC.
3. Another segment of the party this this is betraying a comrade in arms, and fire on the previous segment.
4. The only survivors tend to be characters who found really good cover early on.
The point is that the characters acting in phases 2 and 3 both felt they were role-playing their characters and acting as soldiers, etc., should. They saw themselves acting in an extraordinary way because they found themselves in extraordinary circumstances,
not because their character's psychology was extraordinary. It was only the first character whose actions were completely unreasonable, but the chain reaction only needed one of those to start a cascade.