This happened to the SickRage project a while ago when a developer was not happy about the project being forked. The repositories were down for some time.

https://www.reddit.com/r/sickrage/comments/6oep02/false_dmca...

Actually it could be and it is sometimes abused. You don't need to proof that you are the copyright holder to submit the request. Companies tend to err on the side of removing things to make sure they don't become liable for copyright violation.

That happened before on other platforms, I know about steam and its workshop in particular.