Here's why: The technologies have become simpler and the barriers to entry have become low. Once upon a time, if you knew the mainstream languages (C++ and Java) it meant you paid your dues learning the hard stuff. Today, with nodejs becoming mainstream, and the cloud at the backend, developers who come from an "html programming" background can now build scalable and enterprise grade applications without breaking a sweat.
That leaves the "hard core" dev in a little bit of a fix. "Where do I go from here?" The front end is where the complexity is today. Its hard to compete in the AI and machine learning roles when there are phds doing this for ten years who are on the same table. Devops is also simple enough for someone with 3 years of experience of be on top of the game.
Now add to that the fact that the number of devs with 8 years of experience (what I consider the senior dev) grows bigger and bigger each year..
The next slow down I believe will be brutal to our industry.