I like some of the possibilities Richard Carrier’s puts forward on this:
Saul/Paul had expressed anxiety over how difficult it was to keep to the letter of all of the Jewish laws, it was a source of ongoing stress for him.
Being a persecutor of Christians he would have been familiar with what they preached and maybe even admired how much easier their religion would be to follow than those Jewish laws.
He could easily have begun to feel guilt over persecuting these Christians, who he perhaps felt weren’t really hurting anyone, and that guilt could have mounted over time, gnawing at him.
Combine all those with a long road trip where maybe the sun was a little too hot that day or maybe he hadn’t gotten enough to eat, and boom, perfect conditions for a fatigue or stress-induced hallucination that just so happens to deliver him out of points 1 and 3.
It’s all speculation of course, but still, I could see it.