How do quasars die?
A quasar dies when it runs out of material falling into the accretion disc.
A quasar is material falling into the accretion disc around a supermassive black hole at the centre of a galaxy. The accretion disc is material swirling into the black hole. Material falling into the accretion disc gets heated through friction and gravitation effects to the point where it emits huge amounts of energy. When the supply of material runs out the quasar stops emitting.
Quasars are thought to be young galaxies which have an abundance of material to fuel the quasar. Nearby galaxies are too old to be quasars, but they, including the Milky Way, could well have been quasars in their youth.
When galaxies collide, it is possible that enough material becomes available to reignite a quasar.