The shop would be at fault if either 1) they missed an existing problem which they *should* have spotted--both because it was part of their inspection or review of the motorcycle and because it should have been obvious to reasonably careful mechanics--or 2) they caused the problem while working on the motorcycle. Otherwise, they would not be at fault. You may need another independent mechanic or expert to examine the motorcycle and determine if this was caused by the shop or if, when they were doing the work they you'd brought the motorcycle in for, they should  have seen and corrected an existing problem or not.