Surfaces with a very fine surface finish that are clean and tight together can weld to each other. It's caused by the process of electrolysis. It could also be caused by galling which is a product of friction between the parts. In both cases, the solution is to apply a very light layer of grease or oil(insulator) on the shaft before sliding the bearing on. Most of the time this will prevent future problems. A spray on moly coting also works well when applied to the shaft.