Romney asked potential VP candidates for their tax returns for more than 2 years and if elected, he'll ask for more than 2 years of tax returns from anyone he appoints to his cabinet. But for some reason Romney's CEO mentality leads him to think he can get away with not releasing tax docs for more than 2 years - because he said so.
Romney either did not pay any taxes some years, paid very little, or disclosing his investments would be embarrassing so he decided to take the heat now and avoid what could be a worse disaster for him if he did release his tax docs. The question is, how long can Romney hold out? - as the article said, even Repubs are after him now to come clean.