We won't know until we see them why Trump has been so desperate to hide his tax returns. Given what we know from extensive investigative reporting by the New York Times, his family used tax schemes to avoid legitimate taxes on their wealth. That his business has been dependent on foreign sources of finance for many years is also a clue to his nervousness. So, too, may be his creative use of write-offs and loopholes, or his actual wealth being much less that he's boasted about.
April 23 is the date now established by House Ways and Means Committee chairman Richard Neal for the Treasury Department to produce the personal and business tax returns they have a right under the law to see.
UPDATE: In related news, the New York Times' David Barstow, Susanne Craig and Russ Buettner have won the 2019 Pulitzer Prize for Explanatory Reporting. Here's the citation from the Pulitzer committee:
For an exhaustive 18-month investigation of President Donald Trump’s finances that debunked his claims of self-made wealth and revealed a business empire riddled with tax dodges.Bwahahahaha!