I've seen that happen at my guild too.
We started tbc with a big roster from three guilds merging (not all players of all 3, parts of them went elsewhere).
We knew those numbers would change, as people would quit, reroll spontaneously, be on vacation,...
This week we had to cancel our 25 man, as 2 healer quit entirely, 2 tanks are offline for a month now, one transfered server.
Also we seem to have gotten a lot of friends / family people in, who are below average players and bring in very little effort
(Barely any enchants, jewels or food). If it wasnt for the few good players in here, pushing the others, we probably wouldnt have gotten most of them through the - lets be honest - very easy kara attunement.
We did clear all of t4 last week, but it felt really hard to get this raid through.
I dont see how we get them all through the eye quest chain, or the raid afterwards. It feels really bad, but i dont see a future for this team.
The better raiders will move to better raids and i cant really blame them for it.
The rest will probably take forever to get forward, if they do so at all.
Sorry for the long post and me rambling on.
Tl:dr we lose the better members because this guild has an unhealthy mix of good and lazy players