Gravity sucks.
I'm sure that sudden stop (cab) woke him up.
"I fell asleep" many times and awoke to find I somehow landed upright between two trucks or (somehow while "I was asleep" dropped me trailer at a dock and was parked nearby).
Optional responses:
"I experienced a dimensional flux", "Had to avoid a water buffalo", "Yogi and Bullwinkle were dancing on the highway", "I wasn't driving", "I plead the 5th".