I am no expert, just a rider. Since no one has responded as of this writing, I will happily chime in.
If the rear slides out and continues sliding unabated, this will result in a relatively slow lowside, the same that you expected. If the rear begins to slide even for a moment, then regains traction, that can generate a rotational torque that flips the bike outward in a couple of milliseconds, resulting in a highside. That event is so fast and powerful that once it is initiated there is no recovery.
You pointed out several traction hazards from that day. If too much throttle exceeds the available traction: Once the rear starts spinning, keeping steady on the throttle may keep the tire spinning, allowing one some chance of recovery as the bike sheds forward speed, while countersteering the bike upright. Alternatively once the slide has begun, certain actions like shutting the throttle quickly or using the rear brake can stop the rear tire spin, resulting in the regaining of traction and the bike flipping outward.
I think that giant spring could have hit the bike in a couple different ways causing a momentary loss then nearly instantaneous recovery of traction. It could have hit the rear tire or acted like a kickstand hanging due to a band spring launching the bike upward briefly. Is the spring painted? If you could find such paint on the bike, that forensic evidence might be helpful.
Here is a video of a highside. After the slide begins we can hear the rider chop the throttle, the tire chirp, the throttle rises then chops, the tire chirps a second time, then the highside. The sound and view of the erratic throttle and the tire gaining traction while moving sideways are the hints of why the bike then violently flips.
https://youtu.be/JwlZiArfnYg