Honestly speaking, I'm not surprised, not because it's HashiCorp, because they are moving away from BSD/MIT style license to a much more restrictive source-available style license.
"Open Source" software always have been loved by companies because it provided extreme flexibility (closed forks, forks for customers, secret sauce addition, etc.), plus announced the message that their code is free for all, given there's no warranties.
These companies have skipped more nuanced licenses such as Apache, MPL, EPL even GPL in some cases, trusting that every actor in the software landscape is rational and ethical.
The idea was nice, but it involved humans.
After a couple high caliber forks, HashiCorp indeed felt the pain, and reflexively they moved to BSL.
What they forgot is their initial license has been designed to allow this in the first place. MIT/BSD/Expat is not suitable for monolithic code bases of this size, but people won't listen.
On the other hand, the code is HashiCorp's. They can do whatever they want with the code they write and put out there. They decided to change the terms they share their license, and nobody can say anything about it.
Is it ethical? No. Were the forks were ethical? Depends on motivation.
These things happen when you choose a license without much consideration for the future.
Maybe we shouldn't abuse Open Source software this much and embrace "Free Software" more, but this is just me.
So, at the end, "market forces" abused HashiCorp and, HashiCorp reacted. This is a normal impact/response event. Nothing extraordinary.
Edit: While one may argue that this is also similar with RedHat/IBM, The impact of the change, the number of broken promises, ecosystem dynamics and the motivation behind it make it different, yet I don't want to double the size of this comment.