I was trying to add the "dnd-5e" tag to this question. The system told me that I was making too few changes to grant the edit. Immediately, someone else managed to tag the question, adding no other changes. What's going on?
You had an edit collision.
As you said, someone else tagged the question immediately. That edit had probably already happened before you hit Save. (You just didn't find out about it until a few seconds later, because nothing is truly instant.)
So when you did try to hit Save, the system saw your edit amounted to changing nothing — the tag was already there! — and it complained. As you can probably understand, no changes at all is too few changes to save an edit.