I am pretty sure it is a blend of physiological and psychological factors, and each factor exacerbates the other.
The runners who try to hammer every session to improve miss the point in a couple ways - they are missing the RELAX part of training, where you learn to run more relaxed in training, and it allows you to run faster in races. Physiologically, they are worn down by the time the race comes, and since they care so much, it gets into their head. They get to the first mile on pace, but their legs are already dead, and it all goes downhill from there. They think maybe they didn't hammer their workouts hard enough, and the spiral continues.
Sometimes runners can get away with hammering every workout, but I find they are likely the ones who don't care as much. They don't overthink it, and run really slow on their recovery days - when a stubborn athlete would have a pre-determined pace in mind they wanted to run (they might not hammer their recovery day, but they are not really recovering, either).
The overthinking athlete can overcome these problems while still maintaining their passion, but it usually doesn't come until after high school. I think a lot of times, they don't get it figured out until after college. Once they are done running on a team, they don't get so worked up about practice, and can finally place the right balance of energy on races. Often, this group is capable of running road/track races much superior to what they accomplished running on a team, and continues to race because they feel they have something to prove.
It is the opposite challenge for those who have the right balance in high school - they can flourish through high school and college running on a team, but running and racing on their own takes a different kind of motivation. Additionally, since they were racing to their potential in high school and college, it is tougher to make big jumps. The post-collegieate trend for this group is more likely to be: trains semi-seriously, runs a road 5k a minute slower than their college 5000 track time (likely still winning the race), and thinks, "Eh, what's the point?" and gradually drifts away from top fitness.