They didn't all die out suddenly. During the long time they dominated the Earth's fauna, some died out while others rose to take their places. Towards the end, fewer rose up to take the place of the lost ones. There were not many species around when a) the Deccan Trap basalts erupted, b) the Yucatan asteroid hit c) flowering plants gave them all stomach problems.....

But it would have been circumstances and events that did for them.

Penny