Sometimes it's that the group was underperforming in some way, but no one else wanted to confront the problematic person or people, nor did they want to be the first to leave.

Sometimes it's that people are out of time, or need to get food, or some such and someone leaving is just a reminder of "Oh yeah I should go do those things."

And... sometimes the group can be performing strongly, and wasn't going for long at all before someone leaves and it still happens. That one still baffles me.