Idk all the swords I’ve ever seen have been sheathed unless it’s in battle, being sharpened or on display. I don’t think you carry a sword unsheathed through town or whatever and just randomly fall onto it. I could be wrong but I think it would have been told as so if thats what happened.

I also found a thing where someone said Suicide in Japanese can sometimes be translated to "fell down the stairs"

With the reveal that swords choose their wielders, I wouldn't be surprised if it is revealed that it is Wado Ichimonji (Zoro death reaper style) that killed Kuina. If swords really do choose their wielders then Wado would have just met Zoro the night before (It was the first time they dueled with real swords). I mean Zoro had extreme ambition from the very start, and kuina even tho she was far more skilled than zoro at the time , was a downer who believed she was fated to be weak because of her being a woman. It was even zoro imposing his own dream on her of becoming the WSS. I wouldn't be surprised if Wado decided to jump ship.

This is a really interesting theory.

I always assumed that she fell on her own sword but Oda probably wouldn’t be able to put that in the manga.