I've been catching a few episodes of The Voice lately! ABC's Entertainment says:

Monday night on The Voice, the battle rounds begin, and this season's celebrity advisors will make their first appearances as well.  One of them is Kelly Clarkson, and no doubt her involvement has some fans asking why American Idol's first and biggest star would go over to that show's biggest competition.  Well, her longtime friendship with coach Blake Shelton, whose team she'll be advising, doesn't hurt.   But it basically comes down to the fact that Kelly just loves The Voice.