Why does he stumble when he sees me?

There's this guy who knows I have a crush on him; I'm 100% sure of this. His co-workers from his workplace at our college even know know this. When I appear to be walking and end up seeing him randomly, he stumbles a bit to then continue walking. We have never spoken before and he has done this several time the whole stumbling thing. My female friend even says that when he sees her, he stumbles as well. She says is possibly because he usually see her and me always together hanging out, and so he expect to see me pop up as well. I'm not ugly for one, so I wouldn't understand. Why does he stumble?
Why does he stumble when he sees me?
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