As Trump is attacking Hillary Clinton about her honesty, he should be prepared to show his own honesty by releasing his tax returns. His refusal to do so indicates he has something to hide, and he has already admitted in an interview that the contents of his tax return would lose him the election.
Yes, Donald Trump is likely hiding something by not releasing his tax returns. There are few reasons for a candidate not to release his returns, given the fact that all presidential candidates in recent history have released them. Trump really hasn't given any reasonable answer, like "It's against my principles," or "My tax returns are a matter of national security."
The question is what he is hiding. It could simply be he doesn't want people to know how low of a tax rate he pays, given he has acknowledged he uses every tax loophole there is. He could also be hiding his true net worth, or sources of income. If he had nothing to hide, he would have released the returns long ago just to make the issue go away.
Trump's connections to Russian oligarchs are well known, and it's pretty clear he's also not as rich as he says he is. He has also gotten away with not releasing them because no one has actually pressed him. The media continues to give him a free pass when they could actually soft making it a softball field in terms of questions.