[Joe]

Used to see stuff like this all the time with IPSC shooters in the days of unsupported 38 super barrels. From the shooters description, the key is the case in the chamber was bulged at the bottom rear on firing. It bulged because there was hugely, way too much pressure in the case from too hot a load. As the the ejection cycle started the barrel moves backward driving the bulged case with it and helped by the extractor. (I bet there is also a damaged rim on this case where the extractor tore loose, a big sign of excessive pressure in a 1911.) The bulged case catches on the bullet nose of the next round to feed at the top of the mag. This drives the the round in the top of the magazine back very forcibly until it hits the back of the magazine and the frame. Every 1911 mag I have ever seen has a welded seam up the center of the back. It doesn't have to be rough just a little higher than the surrounding sheet metal. As the loaded round is driven back the base deforms briefly until the primer impacts the seam causing the primer to detonate and set off the powder. A high primer would make this even more likely to happen. Because this case has virtually no support it blows out the weakest spot (the side of the case & the primer), taking the grip panels with it, but leaving the bullet in the case. He's really lucky that he didn't have hard plastic grips or he would be having a conversation with a surgeon to get the pieces out of his hand.

Cure:
1) Double check the weight / volume of the load you are using.
2) visually verify powder level.
3) Seat your primers to the correct depth.
4)If a primer doesn't seat completely right, get rid of it.