World Boss graphic glitch
i believe that is a skill used by one of the class, i am not sure which. and it does deplete CC bar of bosses by a lot so people tend to use it i guess.
If its not caused by a skill, it happens to me too. but i really think its a normal skill
EDIT: just asked guildies what it was and some said it was a glitch like you mentioned
It happens when Revenants use Jade Winds. Has been a bug since HoT was released.