I find it hard to believe that a man continually under audit from the IRS - a situation not unusual for a man in his position - it's strange to make the accusation that he is a tax cheat. Because if he is, the IRS has somehow shirked its duties and let him slide for decades.
The common thread in all of this is, of course, deranged Democrats. Convinced the Mueller Report would fulfill their dream of evicting an otherwise lawfully elected President - their hopes were dashed when it revealed no such thing. So what do they do? Charge Barr with carrying water for Trump and the whole investigation is suspect and they need to see unredacted report. Because, you see, he MUST be a criminal - they are convinced that he must be, even with no evidence.
So - why would they go after his tax returns? Either they're idiots and presume THEY can find evidence of criminality where the IRS could not, or - if there's a scintilla of intelligence there - they hope to embarrass him with nuggets they can dig up. It's why after two years of Mueller not finding anything, they STILL want more investigations, because they're delusional and frantic to find something - because dammit, it MUST BE THERE! SOMEWHERE!
I can only hope the American public is just fed up with them. They need to do their job, and it's no longer finding a Russian puppet. That has been played out.