Why do guys clam up so much around a pretty girl?

I know this girl who is very pretty and yes, she's confident, but it's like she has this weird effect on guys. Every time she walks in the room, guys who were previously relaxed and joking around with their friends before she walked in, suddenly stop and turn all red and just stare at her. It's like they can't function and they get really award. I've seen this with her almost every where she goes, guys she doesn't even know do this. What is it about her that makes these guys act like that? The two of us laugh about it, because it's so obvious. But at the same time, she's told me that she can't just have normal relaxed conversations with guys most of the time, because they act weird. She's mentioned sometimes it makes her feel uncomfortable, so she'll just leave.
Why do guys clam up so much around a pretty girl?
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