I have no idea! There's nothing unusual about riding bikes all the time; just about everyone does it where I live, but she has to almost show off that she's doing it, whereas others like her don't do it.
Maybe she's trying to show that she can actually even ride a bike - looking at how dumb she is, but she probably just wants attention.
It could be the fact that your legs are always spread when you ride a bike. And she's always wearing shorts when she does that.
She's SO annoying with stuff like this. $lut.