Well, two actually.
Confirmation bias.
You see, when you first noticed this, you probably thought to yourself "Hey, that's weird!". And from then on everytime you'd see a woman walking like that, it reinforced your idea that "most women walk like that" because you subconciously ignored the women that walked "normal", since you are used to seeing that everyday.
It's like whne you buy a new car, suddenly you start noticing it alot more around your neigbourhood. Does this mean everybody suddenly bought your car? No, of course not, it's just more promiment to you becuase you "want" to see it.