Why would a guy puts his head down and start biting his lip when passed me?

I have liked this guy for awhile and we have been flirting. He goes out of his way to walk by where I am, I see him glancing at me, etc. We talk, but it is often brief and like he doesn't know what to say, so he makes a random comment and I reply and that is pretty much it. There have been times when he has been around where it looks like he is forcing himself to not look at me, although sometimes our eyes do meet, and other times have stared.

So one day I noticed that when I was walking toward him, he said hi and I said hi back, and then as we got closer and were passing, he put his head down and looked like he was biting his lip a little. I have never seen him do that before and was trying to figure out what that means in regard to me. Any ideas?
Why would a guy puts his head down and start biting his lip when passed me?
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