This will get fixed i the next patch (i think). They wrote something with guardian 4set bonus interfering with Might of ursoc in a wrong way, so use it while you can. Will find link for the bluepost
EDIT: The blue post i referred to seemed to be from patch 5.0.5, already implemented. My mistake

However, i found this post as well:
MMO-Champion - Assault on Zan'vess, ToES Progression, Patch 5.0.5 Hotfixes, Blue Tweets, Fan Art
Doesnt say anything about the druid bug, but mentions warrios being able to do the same with Rallying cry.
Just found the druid post again, and it seems that it is this bug they are talking about. feel free to read it yourselves.
MMO-Champion - Heroic Empress 25 Video, Nov 12 Hotfixes, Win Trading, Blue Posts, HotS Release Date
They are saying that Might of Ursoc is interfering with Deep Earth Vestments in a wrong way. Pretty sure, it's this bug they are talking about.