Cambrian explosion

In the Canadian rockies a fossil field called the Burgess Shale was happened upon. Within it, and in other fossil fields (another one found, I believe, in Australia) was found evidence for an evolutionary explosion. The Cambrian explosion is believed, to not just be an artifact of it being a uniquely well preserved period in the Earth's natural history, indeed, paleontologists and the like, believe that around 500 million years ago there really was an "explosion" in the number and type of organisms inhabiting the planet.
It isn't clear, from my superficial knowledge, what aspect of this ought to be surprising. There certainly was a massive change in the Earth's climate at the time as well (a super ice-age had come to an end not so long ago). So is it just an example of where the selective pressures were "evolving" rapidly, or is there something about the organisms and their genomes at that time that suddenly allowed the exploration of a vast range of phenotypes?

Such questions of course get to the hearts of one of the deepest questions in Biology, as I see them. What are the range of phenotypes within the reach of a given genome. Can we begin to understand the tree of life in a statistical manner?