There have been some suggestions on other forums that the tax returns would show some linkage between him and organizations that are tied to Russian imperialist Putin. I do not know if that is true. However, Trump's release of the records would clarify everything.
Trump's friendship with Putin comes as no surprise. He has said that except for the Middle East he will withdraw the USA from all other forms of foreign intervention. As we all know, Russia has border disputes with Ukraine, Georgia, Dagestan, Chechan Republic of Ichkeria (better known as Chechnya), Abkhazia, and South Ossetia. If Trump withdraws all military from overseas this will give the Russian tyrant a green light to further entrench his imperialistic forces in those countries. That's why tyrant Putin loves him so much.