What Causes This - Possible MP5 problem

This is Winchester 124gr. +P+. Is it caused by too high a pressure forcing the brass into the flutes? We shot about 100 rounds of 124 gr standard pressure and no problems. We then fired 30 rounds of 115, light target loads and were having problems ejecting (not suprised) I ended with 30 rounds of this +P+, and the first 15 rounds went fine with no issues, then we started having failures to eject. Then failures to extract. Upon looking at the brass we found these which were the out of the last 10 rounds fired. Was the chamber getting too hot causing a rise in pressure?