According to the IRS "If you didn’t pay enough tax throughout the year, either through withholding or by making estimated tax payments, you may have to pay a penalty for underpayment of estimated tax. Generally, most taxpayers will avoid this penalty if they owe less than $1,000 in tax after subtracting their withholdings and credits, or if they paid at least 90% of the tax for the current year, or 100% of the tax shown on the return for the prior year, whichever is smaller." https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxes
You would have to look at how much tax was owed/paid for the prior year to help determine why there was a penalty assessed.
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