I know that it's no secret that presidents like Nixon used the FBI as his own Pinkerton agency and that the FBI itself was a haphazardly constructed agency for Hoover's shady shit, but Trump always seems to take it to another petty level that always reflects his own projected fears.
A tax audit on most normal people would cause some anxiety, but as long as they bothered to file in the last couple years and DIDN'T do anything shady like, say, claiming a charity that never actually gave to charity, they'll be fine. A guy like Trump treats an IRS investigation into his tax claims like we're looking at his entire internet history along with webcam shots of his face for when he spots an Ivanka pic.
Also, it's another example of wasted government resources on petty investigation. The amount of money and labor into auditing a worker on a federal income will never turn a great ROI, like say investigating a Senator or tech mogul