Daelo explains that a recent hotfix is aimed squarely at groups piling on with Paladins:

We just deployed a hotfix to modify Holy Wrath so this spell has the same diminishing return mechanics that other player stun spells and abilities currently have. This will prevent Undead creatures that we wish to be susceptible to stun from being permanently stunned if you include enough Paladins in your raid who then rotate their Holy Wrath casts.

This will affect the difficulty of Anub'arak, but we didn't want the "right way" to defeat this fight to involve bringing five Paladins who glyphed their Holy Wrath spell.

In yet another response to guilds/groups who were bringing plenty of Paladins to, dare I say, exploit encounters, Blizzard does some house keeping to keep players more honest. Read the original post and the pages of responses on the official forums.