It's not so much the guys attraction, as it is his confidence and the way he carries himself and the way he talks and how strong his frame is. If he is a really alpha male confident guy with this great charisma and personality, most girls are gonna be shy around him.
Basically, people are shy to people they are attracted to. The reason is this: A biological impulse makes us feel the situation is more serious, the more someone affects us. We react more and we feel more pressure, thus shyness. A boss saying "i hate you" carries alot more weight than a beggar saying it, and in the same way, a hot confident person of the opposite sex walking in the room creates a lot more tension than an ugly boring shy person of the opposite sex.
Girls tend to be attracted to a guys personality and social status/influence/value more than his actual looks, so his personality and social status are thus obviously going to affect her attraction- hence, also her shyness- more than his looks. But they both play a part.