User blog comment:Gourgeist/Who Is The Beast Titan?/@comment-25472766-20141005053628/@comment-5433366-20141007024754

Sorry for the late reply. I'm not sure that completely erases the possibility though.. The one abnormal Titan that ate Ilse Langnar called her "Ymir-sama", pretty much worshipping her.

The Beast Titan was obviously dominant and higher in rank than the other Titans, and remember that the Titans that invaded Shiganshina didn't attempt to eat Bert then. I think some of the Titans have orders to stray away when the mission calls for it, maybe?