Great extinction events happened at both the beginning and end of the Triassic period. The Triassic began after the great extinction at the end of the Palaeozoic era, the Permian/Triassic extinction event. Nobody really knows the exact cause of the Permian/Triassic extinction, but many experts have different theories. Generally, experts agree that many things caused the extinction.
The Triassic had several more extinction events, whose causes are also unknown. The most significant of these took place at the end of the Triassic, which was one of the 'big five' Phanerozoic marine extinctions.