Usure, but probably something to do with the sprocket. Just for the fun of it, I would check it out by grabbing a steel sprocket of the same or similar size, from the middle of a cheap-o 7-9sp cassette (LBS might have a suitable loaner doner), and see if it does the same thing. If it don't, alignment is not the bigger issue; if it does the same thing, it's probably not the sprocket...