This is the explanatation of how it work (not official one, just guess).
There is a limit for the margin call cascade of about 10% drop in 1 minute. Then the algo stop market selling the , and let the buy order to fill the gap. Then it sell again to the previous limit and repeat the processus until it sold every from the margin calls.
I m open to any suggestion or comments
Edit : the begin of the action was due to a single 5k sell order to 575.