The owner is paying himself almost nothing and pulling the rest out as distributions
An S corp only saves payroll tax on the part of profit that gets paid out as a distribution instead of a salary. Reasonable compensation is the IRS term for the salary piece, and it just means a number close to what you would actually have to pay someone else to run the shop, dispatch the crews, and do the hours the owner puts in. Plenty of owners set that number at ten or fifteen thousand dollars years ago and take everything else as a distribution, because distributions do not owe payroll tax.
That gap between a token salary and the real value of the owner's work is exactly what the IRS looks for first when it opens a file on a trades S corp, and a shop pulling six figures in distributions on a salary that low is an easy target.
If the IRS reclassifies those distributions as wages, the shop owes back payroll tax, currently around 15.3 percent, on every dollar that should have been salary going back several years, plus penalties and interest, which on a shop clearing two hundred thousand dollars a year can run well into the tens of thousands of dollars.