The rider is following a car, rewind it a bit before the car and bike enter the turn where the rider slides. Watch the brake lights of the car. They come on to set entry speed and then go out as you might expect. But then suddenly come back on. This alerts me of something ahead. In this case, I think it was just the blind, decreasing radius of the turn, but that alert would have also had me ready for anything else.
Or... I have a bit of OCD when it comes to lights.
So the takeaway here is; if you see unexpected driving behaviour (duh) or unexpected/erratic braking (lights) then be on the lookout for trouble.