Why does Shelby come back?

Sometime in Season 3 because Quinn sees her baby for the first time since she was born. I don't think anybody except the people involved in the show know exactly when but definitely sometime soon :)

She comes back because she becomes a teacher at the school.

She comes back as a teacher and she wants Quinn to be a part of Beth's life

SPOILER ALERT!

Shelby is set to return in Season 3. Her first appearance will be in the second episode, I Am Unicorn. She will be back as a biology teacher and cause trouble for Rachel, Quinn, Puck, and former flame, Will.
~ taken from link