For anyone interested in the origin of life I can heartily recommend Nick Lane's "The Vital Question", which sums up a lot of the most recent research in this area and also briefly talks about the Cambrian explosion. Really fascinating, deeply interesting stuff.
TL;DR: A <i>"small, perhaps temporary, increase in oxygen"</i> happened. Then <i>"the advent of pervasive carnivory, made possible by oxygenation"</i> managed to <i>"set off an evolutionary arms race."</i>