As a former gifted kid, I'll venture an additional hypothesis:

When you're unusually smart, you tend to outperform your peers — so more is expected of you. More work, more responsibility, more rising-to-the-challenge. You're expected to do disproportionate amounts of work, simply because you can. This starts in childhood and is exhausting even then. A lifetime of it? More so. And as you get older, the problems get more complex, and the responsibilities get bigger. People try to "milk" too much out of high-IQ individuals. It's unfair, it's stressful, and it robs a person of time they could otherwise be spending on self-care or on activities that give them a sense of true personal meaning. All in all, it's not good for their mental health.

It would be interesting if follow-up studies presented a Likert scale with the opportunity to respond to statements like: "I feel like more is expected of me than of my peers in the same class/job." "I tend to do a disproportionate share of the work on team projects because my teammates realize that I can do it better."