[Fraxinus]

The Cambrian explosion happened over a period of 80 million years at a time when conditions on earth were becoming more friendly to multicellular life. The major phyla of animals that “burst” into being did so over 20 million years of that period. The appearance of an explosion was enhanced by an extinction event that preceded it.

I read one commentary that part of the reason for the "explosion" was that it was the period when hard skeletons became common, and thus were preserved much more widely. So many people seem to avoid the point that preservation of fossils is not a purely random event.

[JimSEA]

That sounds quite likely. Reference the famous trilobite which developed a hard exoskeleton at the tail end of the explosion.