The first guild I was in back in early BC broke up after BT was released, then my favorite guild of all time broke up when Wrath Launched. Now my current guild is breaking up due to poor management and not enough people. (BTW I consider a guild breaking up when they stop raiding, as that is the only reason I play this game.)
My current guild, has been around since early Vanilla, is at 300 members, and about 25 active players. Our raid leader is a decent guy, and great wow player. But his refusal to talk to other raid members and forcing people do play specs and roles they didnt want to lead to half the guild leaving to make their own guild. I have been in this guild since the beginning of wrath, where we did great but in the end he caused us to lose half the guild several times. He also keeps trying guild mergers, and guess what this never has and never will work out. each and every time we do well for a short time with the new players then all of the sudden bam it all goes south.