[rice_classic]

Do you have other data for a normal lap? The braking application @ 53.5 has the same ramp angle as your panic braking moment @ 59.5 albeit not as hard.

Without video or other data for comparison, the ramp angle suggests you "stabbed" the brake pedal instead of "rolling onto the brakes" and that may have upset the car which can easily happen if one caliper grips slightly differently than the other under initial load. When you stab the breaks that difference between the two sides gets exaggerated and can move the rear of the car. Also the compliance in bushings can cause wiggle under breaking as well or the camber of the pavement.

You are doing over 100mph at that moment and there's a lot of wind under the car as well.

This is all guessing but I've had my tail step out lots of times under braking for a variety of reasons. The face that your lateral load data changes at the peak of your initial braking application and then snowballs from there suggests that it was the braking application (technique, function of the pads, compliance of bushings) that induced this.