I found a YT video that shows the ADSB data. It has the appearance of spatial disorientation. The flight was at night, in IMC. I took Private Pilot Ground School quite a few years ago and got a light/mild introduction to spatial disorientation... The real thing is much more powerful than what I experienced. In short, your "ears" tell you that you're going one way and your flight instruments are showing something else and it's very difficult to ignore your senses and trust the instruments as you're instructed to do.
If I had to guess, the airplane was probably on autopilot and then at some point, the pilot began to experience the spatial disorientation, didn't believe the instruments, and began hand-flying instead according to "feel" instead of via what the instruments are showing. Just my guess, and and it's probably the guess of a few aviation buffs too.