how many people have been charged an "underpayment penalty" who are salaried employees who have been consistently employed with no major changes to employment

How can you be hit with an underpayment penalty when nothing changed on your W4 since last year (owed $34 last year), salary (2% increase) and employment status didn't change... what the hell am I missing? Been trying to get anyone at the IRS to answer that one. I didn't hit a major bracket break...