I’d say it’s also because he’s a billionaire. But, given that he’s a candidate, I’d say that he’s gone from very high probability of audit to politically determined certainty.
Certain people and companies are audited every year based on the size and complexity of the returns.
I worked for a major corporation. The company I worked for was audited every single year. I worked in the finance area. And some of our work was done in such a way as to make sure we had documentation for the audit the following year. We were always cognizant that IRS and other audits were always happening every year. It was just a part of the job.
Trump is probably in a category of business and wealthy individuals who just automatically get audited.