http://en.wikipedia.org/wiki/E...
Cretaceous–Paleogene extinction event
As originally proposed by a team of scientists led by Luis Alvarez, it is now generally believed that the K–Pg extinction was triggered by a massive comet/asteroid impact and its catastrophic effects on the global environment, including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis.
Triassic–Jurassic extinction event
Gradual climate change, sea-level fluctuations or a pulse of oceanic acidification[6] during the late Triassic reached a tipping point. However, this does not explain the suddenness of the extinctions in the marine realm.
Asteroid impact, but so far no impact crater of sufficient size has been dated to coincide with the Triassic–Jurassic boundary.
Permian–Triassic extinction event (the one claimed here)
There are several proposed mechanisms for the extinctions; the earlier phase was probably due to gradual environmental change, while the latter phase has been argued to be due to a catastrophic event.
Late Devonian extinction
The causes of these extinctions are unclear. Leading theories include changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism. The impact of a comet or another extraterrestrial body has also been suggested.
Ordovician–Silurian extinction events
The immediate cause of extinction[which?] appears to have been the movement of Gondwana into the south polar region. This led to global cooling, glaciation and consequent sea level fall. The falling sea level disrupted or eliminated habitats along the continental shelves.
TL:DR -> Maybe some major extinction events were caused by climate shifts, but all were theorized to be gradual shifts, not sudden. The sudden extinction events are generally due to volcanic or impact events.