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.
Some powders are density sensitive, if there is too much airspace in the load they will detonate rather than burn progrssively and pressure will skyrocket. Experimentation is best done carefully by experienced people.