Unfortuantely, unless you're one of the lucky few to be playing in a guild with friends who are also playing at your skill level, and are at a happy progression rate in raiding, guilds can tend to be rather unstable.
I have been in guilds that were together for years, that have broken up overnight for the tiniest of arguement, or because one or two people gquit.
If your class has been known to be nerfed considerably, or you aren't pulling top DPS, it can be considerably challenging to find yourself a NEW guild should something go wrong. And, when you do find yourself a new place to play, it can be difficult to impossible to be considered for gear upgrades because not only are you new, but you're not pulling the DPS that you were expected to be doing, or that your class USED to do.
Unholy DK is, imo, one of the classes really being affected by this right now. The changes aren't live yet, and are bugged on the PTR, but I have seen substantial difficulty getting into a guild at the moment. This could be argued as a DK problem in general, but with little or no raid utility, a lackluster interrupt, and no PvE CC to speak of, losing DPS output is not very attractive to potential guilds right now.
It's easy for this to feel like a slap in the face to a PVEr, when they feel like they must reroll a new class or at the very least change spec and grind out/pay for all new gear in order to continue to raid and be successful.
A sort of companion question to this could be "why in PVE, if it's a team effort, do so many people complain about classes or skills being overpowered in the first place?"