The article failed to mention that Saturday was 'Tax Day' in America; the day that they officially have to file their taxes. Ever since Nixon got into a debacle over his taxes surrounding the partial sale of his California ranch, presidents have released their tax assessments for scrutiny. It is a tradition of over 40 years. Trump refuses and suggests he can not because they are under audit by the IRS. The IRS says he can infact release them.
It is important that he does so for many reasons. The marchers (and they are not alone) want to know who pays Trump personally, and to whom he owes money. The subtext here is Russian influence on Trump and his presidential campaign, currently the subject of investigations in the House, Senate, and by the FBI. Additionally, right now it is impossible to say exactly how billionaires like Trump use the tax system, and therefore how much he and his family would personally benefit if he passes different tax reform proposals.