While it could be that somehow we've got an epidemic of left-handedness that blossomed in the 1920s, what's much more likely is that we've gotten more accommodating and understanding of left handed people, so there was less incentive to fit the mold of right handed folks.
I'm sure there were people who were alarmed at that rise, who felt that we didn't really need left handed tools and accommodations for the "2% of people who are left handed", but when you made those things available, we stabilized at about 12%.
Antidepressants and ADHD meds are, imo, in a similar place -- we've increased our understanding, we've increased our accommodations, we no longer force the people who experience those maladies into the stress of acting 'normal'.
Once you remove demonization, of course diagnosis are going to skyrocket. When it's no longer a scary thing to admit depression, ADHD, or autism, it becomes easier to get evaluated and care for them. I experience depression, I am on an SSRI, it helps me a TON. Historically, I might have been involuntarily committed or been told to "deal with it". I guarantee you I would not be as successful as I am now if I didn't have the support I do.
(Similarly, I expect you see a lot more people being "out" in the LGBTQ+ community as a result of a similar phenomena. Once it's no longer criminal or "get lynched" territory, we see numbers of gay and trans folks increase.)
These things seem like fads. They usually aren't, they are usually the case of a particular mode of people being pressured to be silent and invisible. When you remove that negative stigma, the revert to the mean can be dramatic, but it will stabilize at a new normal level.