They're not crazy, and it only works if they have more political capital than you do. If I was forced to accept substandard work, and had no other choice but either maintain that bad code, or quit, I'd threaten to quit too.
If any of these people has a disagreement with a co-worker over the co-worker's changes, it's their duty as a fellow employee to explain to the co-worker why they disagree, or at least explain to the manager why they disagree with the change, so that the manager can explain it to the co-worker. But just making the change and then threatening to quit... that is most definitely crazy.
> No, they are crazy.
That may be.
> Reverting someone else's changes without explaining why is crazy.
Not necessarily. There are times when you are so wrong you aren't even wrong.
> If any of these people has a disagreement with a co-worker over the co-worker's changes, it's their duty as a fellow employee to explain to the co-worker why they disagree, or at least explain to the manager why they disagree with the change, so that the manager can explain it to the co-worker.
Let's imagine a scenario where they had done this, and yet nothing had corrected the problem. At that point, it would be quite reasonable to revert someone's changes without explaining why, and threatening to quit if one wasn't allowed to.