It seems like every big disaster I hear about occurs when there are huge groups involved. I'm getting into splitting next year, so this is a total amateur hour observation, but that's what it feels like.
Big groups result in various kinds of heuristic traps. Ian McCammon has a great article on the psychology of back country travel that I would recommend to anyone. For example, he found that "people who travel alone or in groups of 6-10 appeared to expose themselves to significantly more hazard than groups of four or groups of more than ten." Although he can't tell us why.