weak bolt spring or late bolt, or both. in Most cases, when the bolt spring is ok, the problem lies in a very critical bolt release, that may let the cylinder overotate.
it can be fixed with an earlier bolt realease, that will prevent the cilinder to acelerate and miss aligment....it did happened to my Uberti as it came from Factory....in my case I adjusted the bolt release to take place as soon as possible after half cock. with a rational margin.