If we look at the timeline of ADHD research, we can see that there was a lot studies about it in the 80s, and even more in 90s, so it kinda makes sense that there would be more ADHD detected in the population when the general understanding of it improved.

Also DSM IV was published in 94, and refined the diagnostic criteria for ADHD https://www.cdc.gov/adhd/media/pdfs/ADHDTimeline.pdf