that's how she got stabbed

It works just as well to say "That was how she got stabbed".

We say "that is how..." because the reason, the explanation, does not change. It is a valid reason in the present, although the event happened in the past.
So it's simpler just to say "That's how...".

or:
'"This is how she got stabbed: ...." as an opener.

That's how she was stabbed.