Why do women walk past with their head down?

Whenever I'm out in public, I notice that whenever a young woman about my age (I'm a 24 year old male) walks past me, often times she puts her head down until she's just past me, looking down at the floor. I may half-notice this person as I would anyone else walking past, but other than maybe a brief glance it's not like I'm ogling, staring or even purposely trying to make eye contact. It makes me feel like I'm giving off a creeper sort of vibe, even though I'm not like that at all- I'm almost always just minding my own business. Why do they do this?