Seems like if the big bang was not the first big bang, and it's a cyclical event, then black holes may become unstable once they run out of matter to consume. Once all black holes have consumed their galaxies, and all the available ISM, and each other, and there is just one, and it runs out of all matter to consume, all gravity is consolidated, all EVERYTHING is consolidated, then a new reaction begins to occur, like an engine that has run out of coolant. The reaction reaches a critical point and an explosion occurs that is proportionate to the mass of the object.
Of course, this depends on the assumption that the Universe will stop expanding, and that the weak force known as gravity will gradually mop up.
Thus spoke The Armchair Physicist.