From what I've been reading, no-one seems to know who decided to drop the price of oil: was it Saudi Arabia and China, to destroy the US Shale industry? Was it Saudi Arabia and the US in order to bankrupt Russia? Was it all intended to collapse the entire system, due to the energy derivatives, or is that just an unintended consequence?
Either way - and it hardly matters now - the still-dropping oil price is making the whole complicated and convoluted financial system come apart at the seams. Here is a pretty good explanation of what is happening, right here, right now:
PS In an increasingly-common act of synchronicity, no sooner had I posted this Post, when this article appeared, giving me the answer:
It is as I suspected.