> I think it must be the shock that comes from being at the top of one's high school, then arriving at MIT and no longer being head-and-shoulders above most of one's peers.
Yup. Some stats for a recent MIT entering class:
97% were in the top 10% of their high school class. 100% where in the top 25%.
25/75 percentile SAT math: 740/800
25/75 percentile SAT critical reading: 670/770
25/75 percentile SAT writing: 680/770
Only about 9% who apply to MIT are offered admission. I'd guess that the ones that get in aren't the ones who had to work hard for kind of scores listed above--they are the ones for whom those kind of high school achievements come easily, giving them time for the challenging projects and activities beyond the standard high school curriculum that made them stand out to the admissions people.
They now have to actually work hard for "regular" classes. Half of them find that their best effort lands them in the bottom half of the class. That is indeed one hell of a shock.