1) not fitting in socially;
2) higher awareness of things.
Neither of these are good for anyone's mental health, but they're not problems directly caused by intelligence itself. Their combination is especially bad, as one could be aware of potential problems while others aren't, generating all sorts of friction.
I believe higher intelligence simply requires better personal mental health management, but there isn't really anywhere for people to learn that and I would say our current philosophical assumptions do not facilitate it.
Here's a recent thread about that: https://news.ycombinator.com/item?id=15886128
Yeah, it's possible that people with higher intelligence are more susceptible to some of the mental health issues that academia seems to induce. I know I struggled with some issues the last ~2 years of my PhD, but the issues were probably already there at the outset.
This might be wrong but it seems like an obvious superficial criticism.
The same is also true in most other areas of higher personal achievement: lawyers, CEOs, politicians, etc are all disproportionately on various spectrums of depression, substance abuse, sociopathy, narcissism, and so forth.
At least part of the problem is how we view mental conditions as a society, which leads people to think that this is abnormal.