Why did this guy's eyes widened when he saw me?

Anonymous
I was just going to my class as a lot of people at the same time. When I turned the corner, I noticed that guy talking actively with his friends, looking in my direction, but he seemed like he was staring into space and quickly glanced at me. Then his face's expression totally changed, he stopped talking, still looking my way, his eyes widened, I felt he was staring at me at my stomach, his eyes then quickly moved to my boobs and his eyes started to move quickly, looking somewhere else next to me (there was no one) then looking again at my boobs.
I wasn't wearing anything revealing. I had high-waisted jeans, a black top (no cleavage) and a long cardigan.

I've already noticed that guy before because one day at break time, I wa standing with my friends and he was standing behind with his friends (facing my butt), when I turned around to see a friend coming I almost caught him looking. Minutes after, I turned around again, he has changed of place and was now turning his back on me. After break time, he was staring at me from afar. Though, after all this I've seen him look a few times but not that much, he usually doesn't look, or before passing and then when passing me, he usually looks out of the corner of his eyes (when he looks).
Once, I walked past him, really close (crowded place) I felt he was staring at me, looked out of the corner of my eyes and he was looking like at my face like he was studying it or maybe following the moves of my eyes and he looked away very quickly.

Why did this guy's eyes widened when he saw me?
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