To me it has to be a cursed sword thing.
We see zoro encouraging kuina here
then this happened
We also see here that certain swords need extreme will/control over them.
So maybe kuina motivated from what zoro said to her tried one of kozaburo swords and did not have enough haki to control it.
It'd be messed up if it was
wado ichimonji that killed her tho
But hey it could've just been that it was an accident with
@stairs-kun