One interesting hypothesis I've heard is that people with mild tendencies toward autism or similar problems are more likely to meet each other, have children, and find that the tendencies reinforce. I have no idea how much evidence, if any, there is for this; I vaguely remember it being suggested because of Silicon Valley's levels of diagnosis.

As for the "overly diagnosed" thing...I'm too angry to go there, and this from your second-hand snippet.