The main scenario is when a guard suddenly enters the area. Let's say you are about to shoot a rope arrow and a guard opens the door. Having the bow out makes you more visible, so you better make sure it is sheathed as soon as possible. There's that moment of panic or need for haste. The sheathe key is pressed once on purpose, twice on purpose to make sure, or twice by mistake. Pressing it twice by mistake and the bow being equipped again would alert the guard.
I played TDM for days using the "put away weapons" key without issue, because I was pressing it once. Then, a scenario similar to the one above happened: a guard suddenly entered the room, I pressed "put away weapons" key twice, and the guard saw me because my bow was pulled out again.
Does that make sense?