What's the psychology behind this girls behaviour?

There's a girl in my class in college (we are all 20 by the way). She is popular with everyone, especially the guys. Shes the best looker in college (and I like her, well we all like her..)

Anyway she's always talking to this perticular group of guys (me included), one of the guys in our group is a little shy and doesn't say much and is hard to invite out and infact is more on the fringe of our group rather than in it, antisocial guy.

She's only said hi to this shy guy, but the shy guy didn't give her much so they didn't connect, but she continued with the rest of us being social,

Anyway weeks later this girl invited our perticular group to a get together at lunchtime (which she hasn't done before)

Anyway she seemed very annoyed and upset when we all arrived, she seemed very annoyed that the shy guy did not attend.

I find this annoying. What is the deep psychology reason behind this?