I assume decently progressing guilds are guilds who cleared normal by now.
I can understand a lot of people quitting because of feeling the need to do LFR (Let's be honest here. Those upgrades are not something to turn your back to when you want to raid seriously or even semi seriously.) and then having to clear and farm normal and then finally do the same raid again on heroic. People burning out on content combined with the problems guilds can have and all the different opinions the members have can cause guilds to disband a lot faster. If you are sick of the farming and then some other problem occurse it is far more likely for people to quit and when your guild is losing more and more people things only get worse with a good chance of disbanding.
What is more fun? New refreshing content or the same content but a little bit harder? I already got sick of normal + heroic in WotLK and now we also have LFR on top of that. I won't be doing LFR in 5.2 and see how I feel about it then otherwise I will just quit raiding all together and just do challenge modes and pvp with friends.