I don't see how being a huge line in the bathroom, may be the cause of girls liking to chat while waiting on it. I'd say they like to chat always and everywhere. I love to chat but, it would be so gay to tell to one of my friends to come with me to pee.
They ain't shy, they are insecure. Like when deciding for a guy (they always take the worst option), or what to order for eating (they change their minds about three times), or what stupid dress and shoes to wear.
OMG!! now i got it..i'm not really girl reacted, no bath room group, i go whenever i want..and i don't like the the noisy chat in bathroom especially if you gossip someone there, you gonna looks stupid more than the person you're gossiping :-P