Following the public protests of April 15 where large crowds gathered to demand that President* Trump release his tax returns for public scrutiny, like every other presidential candidate since Nixon has done, Trump took to Twitter to claim that the "small rallies" (25,000 people gathered in Washington DC, and 20,000 in NYC) must have been "paid for". Then he went further into crazy territory by asking "someone" to look into who footed the bill. Of course, he offered no evidence to support his claim.
Apparently, it is unfathomable to Donald Trump, a person living in his own narcissist version of reality, that people find it highly suspect that he refuses to show his tax returns, and that his continued refusal only makes it look like he has something to hide. Polls show that even a substantial portion of his own supporters would like him to release them.
It appears to be impossible for Donald Trump to realize that he is simply not very well liked, despite being elected with a very narrow margin, losing the popular vote and getting approval ratings which are historically low for a new president. For anyone but him, it's pretty obvious why a person who is ignorant, dishonest, vindictive, small-minded, mean-spirited, egomaniacal, unprincipled, sexist and, quite frankly, stupid, might not be the most admirable person on the planet.