From what I read, the experts are sharply divided on the cause of this mass extinction.
The most singular fact about the End-Permian Mass Extinction is that the entire Earth became desolate for some four million years. The first four million years of the Jurassic were nearly devoid of fossils. There are fewer fossils found in this layer of rock than in the Vendian (Ediacaran) period. In other words, not only was life knocked down at this time, but it stayed down for four million years. This is unlike anything else I have ever read about. I can not even imagine a catastrophe that would take four million years to recover from.