While the overall reason for the mass failure you cite is correct - a cascading failure - the interesting bit here are the oscillations that lead to it.

It looks very much like this was driven by algorithmic volatility trading of electricity spots - overproduction, price goes negative, buys placed, production ramps in response to rising price, price rises, sells placed, production falls due to falling price. The period of the oscillations in the grid seen before the blackout suggest a relatively slow cycle, and what they describe in the report sounds very much like this was an interaction between price-driven supply and real world supply.

It does speak to there being inadequate storage available on the grid to smooth demand and therefore pricing, but it also suggests that in certain conditions a harmonic can be set up between the market and price-driven production with catastrophic consequences.