Get some Burton hardware for the bindings. With that channel setup they have, I am guessing the Union hardware for normal inserts is to long, which is leaving a little room for play even when the screws are tight. That is the only reason that the bindings would move positions the way they are. I am almost positive though that boards with the channel set up require shorter hardware to fight against this. Should be able to get some from any burton dealer for cheap/free.