Or is it a bullwhip effect?

I thought I wrote a reply, but I guess I didn't submit it.

Summary version rather than rewriting: A bathtub curve could show up along with the bullwhip effect. It doesn't describe how it came into existence, it's descriptive of the structure.

If you have a long enough period in your bullwhip effect, you could see the seniors riding it out without replacements or additional hires coming in (they weren't seniors when this started) because of insufficient demand. Then a demand increase causes a hiring surge, but it's mostly targeted at juniors because they're cheaper and you still have your seniors.