Indeed, why? I've seen exactly that. Stupid rules coming up, extremely performing employees leaving or making it clear that unless that's revoked they'll leave.

My favourite example of this was when one company tried to issue all the devs with new contracts with the "standard" IP clauses claiming ownership of everything we did. Since there were a number of Free Software devs on the team, this was unacceptable ... so, while asking for clarification and a change, we simply didn't sign the new contracts. There was no way the company could have taken action against us without completely wrecking their ability to ship, so it stalemated until they backed down.

These are usually the first clauses I query ;)

This works pretty well as a response:

"Okay so I maintain some open-source stuff, it's all on Github. I appreciate you want to maintain your IP, and I agree to give you first-refusal on anything new before I make it public, and stay out of any market where the company has an interest."

This is usually enough to swing things from "we own everything" to "we own everything you make on work time, unless it intersects with one of our products".