Maybe it has Herridge spec settings??. He loves to back it in to corners.
Always hard to say from incar, but does seem to suggest a lock up at the rear....but that could be a whole combination of things... changed grip level (oil,gravel etc) ....car unweighted unevenly by braking and turning in, camber on road, pads in rear biting harder than fronts, rear tyres maybe not up to temp.... just a shitty annoying crash where it could have just been a little spin, but ended up worse...although as other have said, could have been much worse.... is certainly one example of where there is no shame in having ABS in a tarmac car. Good to see you are already embracing the upside and looking to further improve the car.