My first guess would be that the case wasn't trimmed properly if it was just one (similiar to your round). The crimp on the seating die bulged the case. Ask me how I know what that looks like. It could also be that the seating die wasn't set properly and the factory caught the other rounds that this happened to.
Yeah I think you're right. The telltale score line just above the bulge looks like a sure indication of improper trimming...