Raiding is definitely a guild killer. And I mean raiding in general. Raiding too much and not raiding enough have both been the death knell for guilds around the world.

My guild was almost killed early in Wrath because of raiding. We had just enough people to run 10-mans, but it meant everyone had to show up in order for the guild to raid. It proved to be too much pressure, so we went back to being non-raiding for the good of the guild. Everyone who wanted to raid was encouraged to keep their eyes out for other guilds that needed pugs, though, and so far only one person has left for greener pastures (though that was also because he lives in a different time zone, so he was almost never on when everyone else was).

Fast forward a couple years, and I'm running regular ICC raids as a permanent pug for another guild. It was definitely a casual raid group, though we were doing pretty well (we cleared the first four bosses in the group's first week, when several members had not even been in ICC before). As we got deeper into the raid, however, there was some dissatisfaction over our progress. People (myself included) had real life issues crop up from time to time. After our fast progress in the beginning, it took us a couple weeks to get Festergut and Rotface, then a couple more to get Dreamwalker. We almost had Sindragosa down, too, but then half the raid group transferred servers, completely gutting the guild. In an interesting coincidence, it happened the week after I got my fourth tier 10 piece.

I have never seen any other aspect of the game kill guilds as efficiently as raiding does.