This was my interpretation, the largest downward movers across the board were the Microsoft stack technologies.

Perhaps, given this data is from a Q and A site, these languages have just gotten so amazing in the last three years that developers are collectively asking fewer questions because they're running into fewer critical / main sequence blocking issues. Maybe these languages have moved to only being deployed by established and professional shops, leading to less of the entry level developer "noise" questions on SO

[e]: sibling comment explains these trends, quite convincingly, as site demographics.

I'd love to see this data compared to a similar set from github. Although public repo stats would suffer the same selection bias as SO posts.