Last summer my son and his wife were flying in to Pittsburgh from Seattle. We are NW of Pittsburgh and many planes coming in from that direction go right over us. I was able to see their plane flying over.
Lots of things could happen but that's probably pretty low on the list. With a dive that steep, then somewhat of a recovery, then again diving, it reminds me of the Alaska air flight about 20 years ago where the horizontal stabilizer trim system failed. The rear elevators failed putting the plane in a steep nosedive. At about 10k altitude they sort of leveled off by rolling over and flying upside down and getting the nose back up, but the elevator failed back the other way and it rolled and dove again. Kind of like split S but the roll was done in the middle of the loop not before. Sad situation for all on board but honestly the G forces and loss of altitude probably knocked most of the passengers out. They lost almost 18k altitude in 42 seconds. About 425 feet per second.