I can get the bolt being hard to close due to not pushing the shoulders back enough. But I'm not sure that would account for the extra pressure. I guess possibly if the case could not move forward at all it could drive the case into the bolt face. Did you fire some of the other properly sized rounds after this? The other thing I have seen cause pressure out of the blue on a previously safe load is a carbon ring. It will build up and then suddenly your normal load is showing pressure signs. This can be dangerous if you are already running really hot.