What could have triggered mass extinctions of species in the past ?

Explanation

Nobody knows the real reason but the scientists believe that any one of the following could have triggered mass extinction of species in the past.
(i) Fall of sea levels.
(ii) Change in temperature (freezing or warming).
(iii) Asteroid/meteorite hitting the planet.
(iv) Poisonous hydrogen sulphide emissions from the sea.
(v) Nova/super nova/gamma ray burst.
(vi) Plate tectonics.