It's because of hawking radiation which is just singular atoms slowly escaping the black hole, they will become massive in about 100 trillion years, so it will take another 10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 YEARS for all the atoms to escape. After it lets out all its atoms, it makes a GIANT EXPLOSION.