There is evidence that the Rancho La Brea tar pits are a site of rapid burial and subsequent fossilization... and now we find the same for Burgess Shale.

"...the journal Nature reports the discovery of the first fossilised creature to actually be caught in the act of shedding its exoskeleton."

This cannot happen slowly, as:

"There are two main reasons why the fossil record has, until now, been reluctant to provide such evidence.

Firstly, a creature in moult has a soft body, which does not preserve easily - the conditions have to be just right. And secondly, the process of moulting is usually very quick, meaning the "window of fossil opportunity" is small.

The likelihood of capturing such an event is so astronomically small..."

This means it had to be buried rapidly and deeply because the moulting process occurs so quickly, and during the process the animal has a soft body and is easily destroyed.

Quick burials = fossils = cataclysms