I've noticed this too. I was walking down a supermarket aisle today and there's one woman transfixed with looking at something on the shelf. Plenty of room to walk behind her. What does she do the second I'm about to walk behind her? She steps back right in to my path. I mean I'm the only other fucking person in the whole lane and I've only been walking on the same trajectory for the last 20m, and she still manages to get in my way.
My missus has a habit of walking backwards before turning around. I've since noticed that a lot of women do it, but not many men.
Turn on the spot ffs, or at least look behind you before you move!