What’s tweeking our DNA to mutate?
Icky food and toys from China.
Clearly, in the industrial age, we're exposed to all kinds of chemical compounds that people a couple of centuries ago could never have encountered. Also, things that would formerly kill babies either in utero, or shortly after birth, are being "corrected" for, and the genetic underpinnings of those things are being passed on to the next generation.
It can do that all by itself. The copying isn't always perfect. But if the question is, why dis this becoming more common now, I think the answer could be, in part, that women are waiting until later in life to have children. The older the mother (and some research suggests, the father), the higher the risk of genetic problems like Down's Syndrome. It would seem to make sense sense that other issues are occurring, too.