the fixing bolt came loose and the arm came off. As to why, it probably wasn't torqued to the correct spec which the shop is responsible. It will be difficult to prove. If you ask them nicely they will reattach the arm and hopefully dish your wheel for you.
I have had similar happen but it was on a POS FSA crank. Shimano stuff just works. I doubt the machine surface you are seeing is a defect but I don't have dura-ace arm to compare