Throughout history, there have been many periods of mass extinctions. The most famous is the one that happened 65 million years ago between the Cretaceous and Tertiary epochs when the dinosaurs became extinct. The accepted reason is that an asteroid or comet about 10 km in diameter landed in the Yucatan Peninsula in Mexico. The ejected debris from the impact blocked the sun for several months cooling the earth sufficiently to kill of half of all life forms on earth. It also allowed mammals to rise up and eventually dominate the earth (i.e. us).
"The Great Dying" happened 250 million years ago between the Permian and Triassic epochs when ninety percent of all marine life and three quarters of all land life vanished. New evidence published in two articles in this week's Science, postulates that this extinction was not consistent with an impact. Ward et al. showed that the extinction happened gradually over 10 million years with a final push of 10,00 years. Grice et al. find evidence that the extinction was due to a combination of low oxygen and sulfide toxicity associated with a disruption of the carbon cycle and sulfur cycle. They show that the sulfide came from bacterial reduction of sulfate. The source of the sulfate is not known but may have come from volcanic eruptions.
In interviews, Ward has postulated that global warming due to massive eruptions of volcanoes filling the air with greenhouse gases leading to a runaway greenhouse effect where trapped methane in the oceans is released by the warming temperatures is the culprit. This is a plausible hypothesis but there is no solid evidence yet as far as I can see.