They fell asleep, all veteran pilots know that’s exactly what happened.
Lucky that they fell asleep over Wisconsin with their transponders on so that the tower could re-route other traffic.
Imagine if they were approaching Philly, then flew into NYC air space asleep for 600 miles.
If they fell asleep they’re in even bigger trouble, because they told investigators they were having a heated discussion, and the voice recorder will tell the truth very quickly. I really doubt they were asleep, because they were certainly wide awake by the time they were explaining things to investigators on the ground, and they would have realized that any story they gave would be compared to the voice recorder.