"All beings in the universe are different. Humans are so dumb that they think heart disease is the leading cause of death, when in fact it's the almond milk... not even the gray's touch the stuff."
- Resident Alien (TV Show), "Homesick".
Jokes meant to impart some brief levity aside, Until there is a serious amount of independent resources put towards a study of this, I don't think anyone is going to find out what's causing this.
This has been going on for a long while, so I associate it with an ongoing coverup of the facts that are too dangerous to disclose.