For exactly the same reason I'm a Brighton fan.
Her Dad, who was from Salford, is a Man Utd fan. I was born in Rochdale, my Dad was from Brighton and i'd have been raised a Brighton fan even if we'd remained up north.
I don't think Reily is a good example of a plastic fan if you are raised to be a fan of a club you've got family ties to etc.
Zoe Ball is a much better example of a plastic, started out supporting Liverpool, and then changed to the red mancs, when the scousers turned crap.
I don't know who she supports now.