Why is it my responsibility?

Just a kind of funny story from the grocery store yesterday. We all went, so I had Scarlett in the sling and Matthew in the stroller while David took care of the buggy. We had stopped to get something on one of the aisles and someone stepped in front of the stroller. When I was ready to move on, I backed up just a tad so I could walk around them. She decided that was a perfect time to BACK up and backed right into the stroller. She said 'excuse me' and I replied 'no problem' and I thought all was fine. Then I overheard her say to the guy she was with "She really needs to watch where she's going." One thing I can't stand is someone who will not say what they think to my face, and I hate passive agressive behavior like this. She obviously wanted me to hear her. And I don't know WHY I give in to this sort of behavior, but I did...I turned and said "I was watching where I was going, you were the one backing up into me!" and continued on down the aisle. Then she had the nerve to tell me if I saw her backing up I should have said something. (Which I did not see her backing up until she actually backed into the stroller, otherwise I would have said something, but that's beside the point.)

So of course I had to ask her why it was MY responsibility to watch where I'M going and also to watch where SHE's going? What the heck? Can someone explain this to me? She couldn't, and I just have to say I HATE stupid people!