In a relapse of how the period began, global warming brought the Triassic to a close 200 MYA. Higher global temperatures increased storminess, roiling biomes.
There were major terrestrial ecosystem changes. Fires raged through forests. The level of atmospheric CO2 shot up.
20% of marine species went extinct. The reef ecosystems that formed during the middle Triassic were decimated.
Shifts in the lithosphere – plate tectonics – initiated volcanic activity and a massive methane release from the ocean abyss, via clathrate hydrate: a lattice of water molecules storing trapped gas.
Methane is a much stronger greenhouse gas than carbon dioxide, and so would have effected rapid climate warming and ocean acidification. Volcanism and extensive lava spills fortified the effect.
Further, an asteroid strike may have occurred that was causal or contributing. A massive crater in western France coincides with the extinction event.