Why did she react like this?

This girl started my work place in October 2013, but I didn't begin talking to her until around the April/May 2014 because she worked in a different department and I keep myself to myself, any ways cutting the waffle, after a few short conversations I asked her out in June and she turned me down, she was busy and I was like cool, no drama's and we remained pleasant to each other, neither of mentioned what happened, I put it behind me relatively quickly and that was that. Well she left last September and on her final day she came to visit me to say good bye and we hugged and it was the longest and tightest hug I've experienced, it was like a bear hug and so we went our separate ways, no hard feelings.

Well a few days ago, I was out and about, taking a stroll and as I entered a shop to look for some new shirts, I literally looked up, made eye contact with her and just as I registered it was her, I saw she looked straight down at the floor and made way towards the exit. I know we're not friends and we hadn't seen each other for months but I felt it was a weird reaction, I mean if she just ignored me by looking away, I would have been like, okay, cool, but that got me thinking why someone would act like that...

She sounds shy and her "flight response" kicked in so she fled to avoid having to make small talk with you.