With an event like that and only 20 round remaining I would pull the bullets on the remaining loads and perhaps weigh a few of the charges. Though it might only be the one round that may have been overcharged, it is not worth the risk to your safety. I have no idea as to the cause, hard bolt lift and deformed case head indicate the pressure on that round was borderline dangerously high and needs to be looked into. A couple of things come to mind as possible causes, case length (might have been overly long when chambered it may have caused the mouth of the case to crimp) also it might be possible that the bullet was touching the lands or actually in the lands causing and overpressure condition. Might be worth checking the overall length of the remaining rounds before pulling them. Also check the case length, do you trim often and/or check length prior to loading?
