I agree that a lot of it is preparation. My D had 4-6 hours homework/nightly through HS (and more on weekends). She was used to having to be very scheduled and diligent to get everything done. Those skills transferred over to college.
It can also be a shock for students in very competitive programs where everyone around them was also the top of their HS class. Not everyone is going to get As, especially in intro classes where things are graded on a curve.