Lots of possibilities. Wrong powder, double charge, too light of a charge, no charge and the previous bullet stuck in the forcing cone, crimp too light and bullet set back. Too much crimp is unlikely as it would be almost impossible to crimp so tight as to cause that much damage. Could have loaded the wrong ammo. Cylinder didn't index. Too heavy of a bullet for the powder charge. Barrel obstruction.
It blew up and out so his hand was ok. The top strap broke at the front of the frame, peeled back and broke off at the rear. That's what hit him in the face.
I load on a progressive, don't weigh every charge, but I do look in the case as I seat the bullet. Have loaded several hundred thousand rounds with out incident. Did blow up a .22 in Jr. High School but that's another story.
Have fired a .40 S&W out of my Les Baer target .45. Blew the mag out and the grips off. Have had case failures in my IPSC gun when it had a barrel in it with an unsupported chamber. That was common when the .40 first came out.