What event may cause a sudden increase in the diversity of the fossil record, other than a sudden increase in the diversity of life?

A sudden mass extinction such as a volcanic eruptions on a global scale (such as the Siberian Traps) may cause a sudden increase in diversity of the fossil record.

Explanation:

Previously unrecorded animals which were either eaten or decomposed over time, now may show up in the fossil record. Being buried in a layer of ash.