Other than you know…. he’s under an audit from the IRS.
“That said, as a tax attorney, I don’t know that I’d be so quick to advise that he should release the returns to the public. Once the returns are made public, they’ll be picked apart — by Cruz, by Rubio, by the media, by Clinton, by Sanders, by the very public he’s currently wooing. And even though there might not be anything remarkable on those returns, having any potential flaws highlighted during an audit makes the returns a little more difficult to defend. While the audit — which Trump calls “routine” — might be focused on, say, dividends or related entities, it can’t help to have the public asking questions about foreign accounts or charitable contributions (which I fully expect to happen). In that regard, while Trump could release those returns, if I were his tax advisor, I wouldn’t want him to.”
Taken from this Forbes article on the subject.