More of everything
Expanding the roster used to be a very valid reason to permanently have "LF Resto Shaman" on your recruitment page, but the days of stacking as many Bloodlusts as possible are past. Indeed, with the advent of dual specs, flexibility is less of an issue than it ever has been; you no longer need to bench key players just to get a specific buff when they can respec at the drop of a hat.
Yet, there are still reasons to ensure you have at least one of everything, and several of most: first and foremost, gear and buff coverage. If your entire healing roster is Holy Priests, you can probably manage most fights, but losing the buffs and utility brought by the other classes will hurt, and all that spellpower plate will rot.
"Bring the player not the class" still works to some extent, but you never know when you're going to need to stack something, so most of the hardcore guilds I've seen still retain some degree of class balance. During Naxx, this was entirely unimportant beyond having at least one priest for Razuvious-25, but in Ulduar guilds are realising that perhaps they're running low on certain classes and roles, and recruiting to fill those gaps.
Goodbye and all that
Far more than this, though, guilds are recruiting because they're losing players. Churn is at an all-time high for some, with guilds that haven't seen a new trial in months suddenly get several at once.
Why are people leaving, though? At the very top-end, where there are few places to go, players are going casual -- unable to cope with the raiding hours required, a few weeks in, to defeat the harder hard modes, people are dropping by the wayside and relinquishing their raid spot to someone who can always be there. (The jokes about not having a life at the top are, unfortunately, grounded in some truth.)
Then, all the way from the most exalted guild to 'Dragon Warriors of Azeroth*', people are leaving because the grass is greener. Don't like hardcore guild #1's raid leader? Maybe identical hardcore guild #2's will be slightly nicer/nastier! Caused a bit of drama and want a fresh start? Eyeing up that guild a few rankings higher on WoWProgress? Or simply dropped out of raiding and frustrated at the guild you joined to get back into the swing of things? These moves of similarly skilled players from guild to guild keep things flowing.
Of course, the grass usually isn't greener, and yet the myth perpetuates. If you really aren't happy with your current guild, definitely do seek out a new home -- I've been in that situation, and ultimately it's your own happiness and subscription fees that suffer. However, I'm definitely seeing people with minor gripes pack everything up and seek a home slightly higher up the mythological ladder of e-peen, only to find nothing's really changed.
Watching for trouble
One problem is that despite the reputations of certain endgame guilds, not a great deal is known about specific players within the guild, nor its internal dynamics. An applicant to a similar guild gets an instant thumbs up because they're clearly good enough to be in a well-known guild in the first place, but their reasons for leaving could be complete fiction, or a misrepresentation of the situation. This means that as well as good, but unhappy, players, troublemakers also circulate among guilds.
How to spot and avoid them? Firstly, check out their story. This isn't the easiest trick in the world, but guilds are incestuous enough that you probably know someone who knows someone who will have a word or two to say. If not, try whispering an officer blind, but be really careful about this -- it'll clearly depend on the situation. If the player didn't give a reason for leaving their guild, or mysteriously went casual and is now in a 'lesser' guild rather than being re-accepted in their big-name guild, fly those warning flags and quiz the player in depth about it.
An easy way to spot a potential problem transfer is to look at their history of guild- and character- hopping. This isn't to say that every player who's been to multiple guilds and rerolled is a troublemaker, but sometimes you can unravel a complicated story into a pattern of loot-hogging, rerolling, loot-hogging, rerolling... Not that this pattern is entirely uncommon among guilds, but you ideally don't want to let someone in who you know is going to take as much loot as possible and then demand to reroll or else quit the game.
The most fundamental thing here is not to judge the book by its cover. Just because someone has been in a guild you've heard of, whether that's on a world or server level, doesn't mean they're going to be a good fit for yours.
A change in dynamics
Another problem that's causing churn is the fact that raiders haven't really been tested since Sunwell Plateau. A fair bit of your roster from Sunwell might be intact, but over the course of WotLK and Naxxramas real life, jobs, babies and boredom have all happened. You've probably got a good idea of your raiders' idiosyncrasies thanks to fights like Sarth-3D, but there are still unknowns, and Ulduar has shown some cracks in otherwise-perfect facades.
Whether it's a problem with your raider's attitude -- maybe they get frustrated after an evening of wiping, and take it out on everyone else -- or skill, there may well turn out to be players in your roster you could do without. This in turn leaves gaps for new players, equally untested.
It's a gamble, at best, whether a new player will turn out to be better or worse than the departing one. Testing them alongside the old player and gradually benching the old player in favour of the new, if they do turn out better, is a cruel but oft-used way to resolve the situation.
However, it can take weeks to really get an idea of what a raider's like. From a guild point of view, you want the best possible roster for difficult kills, so your newer and unknown players will be benched in favour of old ones with known skill. This in turn leaves the new players frustrated -- often one of the problems they wanted to get away from was being benched! It also means you can't really test them at the 'cutting edge', and simply perpetuates the problem of the unknown. On the flipside, you can't bench your oldest, trusted raiders for a newbie, especially if particularly shiny loot is going to drop.
Recruitment on progress causes all these headaches, and more. Higher profile due to kills and achievements leads to more applications, which require more time to deal with, research and interview, which you don't have because you're raiding. Desperation for specific classes leads you to grab the first one that comes along just so you can keep raiding at the same pace. The problems caused by churn in themselves put off new players, who hop guilds and leave you back at square one.
The only people who can stop churn are the guilds themselves. Find creative ways to raid without having a set roster of five shamans; allow those who can't keep up with the progression pace to raid at a more casual level; don't make ill-advised, hasty recruitment decisions that only worsen the situation. Sadly, most of these concepts are alien to endgame guilds, but we can still hope.
* I made this name up. I'm sorry if your guild really is called Dragon Warriors of Azeroth. No, really, I am.