Sorry this got so long. I think the co-worker not pulling her own weight is the observable thing here. But you'd have to get inside her mind to know if she's being passive-aggressive or not because there could be other reasons for her not pulling her own weight. For ex. maybe she's not able to do her job well because of some kind of infirmity or maybe she's just lazy or maybe she's indifferent.
This is a whole different situation but a new-ish friend once told me I was "passive-aggressive." I was actually following a thought out plan to get rid of her because I found her exhausting and I chose not to discuss it with her. (She was the type who I expected would just twist everything I said and talk about me behind my back anyway). So I distanced myself from her, not returning calls and turning down offers to meet. I was waiting for her to just move on.
Like Miss Manners says, the polite way to turn down unwanted friendship in the same way you'd turn down anything else you're offered that you don't want, simply, politely, repeatedly, decline.
Well, we did have some kind of connection at that point so I guess it wasn't exactly like Miss Manners said lol.
But to me, in that situation, it came down to her thinking I owed her an explanation and that it was dysfunctional on my part "passive-aggressive" not to give it to her. And me not thinking I owed her an explanation and wanting to get away from her, not in deeper with her in any way.
So... I guess what constitutes "passive-aggressive" is also somewhat in the eye of the beholder?