I'm a recent music major that graduated June of 2012. While I agree (and certainly have heard of!) there are definitely students who self-select themselves into med school (my dermatologist being one...majored in Harp at UCLA, knowing that her parents expected her to end up in med school), I do think the ones who have been studying music all their life have certain things shaped into their personalities that may influence how they handle the coursework and how they think.
At least in classical music (realm of my knowledge), there are so many complexities one has to understand while practicing. Yes, we do rote practicing and repetitive movements, but also some really deep analytical thinking. Why does this phrase work this way? There's a very scientific way of practicing (if you're doing the most effective practicing you can be). Many times, one must ask themselves " If I do x...then what will happen to y". You must be able to find the root of the problem. How? Limit confounding variables to figure out everything you can. Can't play a passage? Slow it down and figure out could it be your left hand? Your right? The angle of your elbows? The way you're standing? Well focus on each one and figure out where the problem is. For me, studying was a relief and a break from practicing. Studying would turn my brain into mush, but at least my body was intact. After practicing 6-8 hours, my brain was mush, and my body was done for the day.
For one, at least as a violinist, many of the violinists are just as bad as the typical pre-med gunner you'd find around. Deep down, many musicians (that I've met..at international festivals, school, competitions, etc) are hyper-competitive. I've never met a good violinist that wasn't inherently competitive. Sabotage happened quite often with us, as well as trying to make our peers look bad. For better or worse, playing an instrument fosters a need for perfectionism and neuroticism.
The biggest reason why I ended up pre-med (yeah...I definitely didn't exactly put myself with a more relaxed group of people), was the depressing aspect of practicing. Practicing isn't the same as playing and enjoying it. Typical saying for violinists was, "If you think you sound good, you probably sound horrible". One day, as I was getting ready for graduate auditions, something snapped. I'd be pulling 6-8 hour practicing days, on top of lessons, classes, etc etc--and yet being trained to think that no matter what, it will never be good enough is REALLY depressing. I think the desire to achieve the standard of "perfection" in our instruments makes us really big overachievers.
I always thought that, at least during the pre-med and MS1 and MS2 process, a graded system will tell us what is "good enough". I always saw that as a relief...since as long as I pull an A, it's "good enough". (Now.. MS3 and MS4 will probably go back to subjective grading..)