At some point your fire went out. Probably due to an overnight cook without monitoring your hopper had a "dead space". This is when pellets stop falling into the auger. Then the fire goes out and eventually more pellets slide down into the auger. Since the flame went out it keeps feeding to get to temp. But there are still hot coals underneath. Eventually it will catch fire and when it does the door blasts open and causes a hot fire.
This is purely my opinion and I could be wrong but mine had a flame out once when pellets didn't go down the chute. Then a huge plume of white smoke appeared so I opened the lid. Then it popped when the fire caught but since I had the lid open it helped it not to explode. Then temps went over 500 and it eventually shutdown when it got too hot.
For this reason I do two things:
On my Copperhead Series 3 I keep a long stick handy. About every two hours I put it in the hopper and move pellets towards the hopper. This keeps is from getting the dead space.
Secondly I NEVER do overnight cooks. They say these are set it and forget it but I'm not taking any chances. If I do a long cook I just wake up about 2:30am and start by 3am and just sit outside or watch TV.