Guy's Behavior

Why did he react like this when he saw me?

I had a fairly young professor last semester. He looked to be in his 30s. I’m a girl in my early 20s and everyone loves him because he’s super cool and chill and smart. I consider him one of my favorites and I honestly see him as a TA or something. Last semester he pulled me aside after class before to ask what was wrong and could he do anything to help because I wasn’t talking out in class as much that day and I found that to be endearing.
I dropped one of his classes this semester and sent him an email letting him know and he told me he hopes to see me again in one of his classes in the future.
I was in his office department going to see my advisor and I saw him walking and he was going the same direction. When he saw me he immediately raised his eyebrows almost like he was surprised and I smiled and waved at him and he waved back and then hurriedly went into a nearby room.
I thought the exchange was a little awkward. Why would he do that? I’m not scary looking lol I’m a cute girl. Does he have the wrong idea about me or something?
Why did he react like this when he saw me?
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