Why are there mass extinctions?
Life forms evolve and adapt to a specific environment. When conditions at the surface of the Earth (temperatures, atmospheric or oceanic compositions for example) change faster than the rate of evolution, species disappear on a large scale. They are soon replaced by new species more adapted to the new conditions.
Mass extinctions happen because of climate change, asteroid impacts, massive volcanic eruptions or a combination of these causes. One famous mass extinction event is the one that lead to the extinction of dinosaurs, 65 million years ago. This event seems to be the combination of massive volcanic eruptions (the Deccan Traps) and the fall of a big meteorite.