And then I notice that Relmstein has today posted a nice article about this very subject. He says there's two reasons guilds break up: Either the guild leader decides to quit and take everybody down with him (or her), or guild drama creates a big enough rift in the membership that eventually everyone wants out.
In the first scenario ("death by self destruct"), there's one leader who's convinced there's nothing that should happen in the guild without their say. Weird decisions are made, and ideas or help are refused by the leader. Eventually they decide it's all over, and a few /gkicks later, it's all over but the crying.
In the second ("death by divide"), one or two members of the guild gets angry about something that happens, and starts fracturing the guild apart (usually by gaining power and then abusing it in the eyes of the leader). This one is much more fun-- there's usually a few spirited in-game conversations, a few long rants on the message boards, and eventually everyone /gquits away, as Relm says, "quietly in the early morning."
Is there any way to stop it? Relmstein doesn't give any answers to that one, and in my experience it's completely true that most disbands fall along one or both of these lines. But maybe that isn't so bad-- a guild isn't really a family or a job, even though it has qualities of both. It's just a voluntary grouping of people with the same interests in game. One guild breaks up, another comes along. And either way, it's always fun to watch.