Talk about putting too much powder in your basement reloads!
bore and slug problem... the force has to exit the barrel. If it cant then it goes up and out!
It could also have been too LIGHT of a load. With an under load you get a partial ignition which will unseat the bullet and jam it into the barrel. The obstruction will in turn cause a steep rise in pressure which will increase the rate of burn of the remaining powder. The cylinder explodes and shears off and ignites the adjacent rounds.
The explosion in one chamber triggered the explosion in the other.
Unless you’re just kidding, only one chamber exploded.