It's a common occurrance with 1911 pattern semi-autos. Most people run the dented case into the mouth expanding die first to push out the dent, then reload as usual. The cause I believe is cases striking the lower part of the ejection port as they are ejected. Some people have gunsmiths lower the port to reduce the occurrance. I've never thought the expense was worth it. You brass is just dented and not seruously damaged. It can be reloaded safely.