There are several reasons why Trump might be hiding his tax returns:

First, it’s possible that Trump wants to obscure his charitable giving patterns. “He has claimed he is a very generous philanthropist. The tax returns would demonstrate whether or not that’s actually true.”

The second possible reason is that Trump may have lots of money parked overseas, and if so, this would come out in the returns. “It could be embarrassing.”

Third, it’s possible that Trump’s returns might reveal that he isn’t quite as rich as he likes to claim he is.

There’s going to be a lot going on in those returns, even if it is all legitimate. But we have no way of knowing whether it’s legitimate unless we can actually see them.”

https://www.washingtonpost.com/blogs/plum-line/wp/2016/05/11/why-wont-trump-release-his-tax-returns-here-are-a-few-possible-reasons/