Why do girls look at the floor when walking past?

I started a new job recently and am sat facing the doorway to the whole office (big office with like 40+ people in) and the only drinks machine/kitchen are in my eyeline because of where my computer is.

2 girls look down when walking past me. 1 I've met before at a friend's birthday party (it was that friend who got me the job). I spoke to her then and maybe made a comment on facebook since then but otherwise no contact (for about a year). We've not said hi to each other or anything and she's been around my section as she's friends with my coworkers, it seems like she's pretending not to know me. We didn't fall out and we barely know each other (is that why's it awkward? not quite friends but not exactly strangers?). Or am I imagining that she's looking down when walking past?

The other girl's just a girl who walks past a lot to get to the drinks machine. I don't know her but she always looks down when walking past. I don't know if it's specific to me or just because she doesn't like being in someone's eyeline but she does it (looks at the ground).