Estimated tax law

From IRS website:

https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxes

Taxes must be paid as you earn or receive income during the year, either through withholding or estimated tax payments. If the amount of income tax withheld from your salary or pension is not enough, or if you receive income such as interest, dividends, alimony, self-employment income, capital gains, prizes and awards, you may have to make estimated tax payments. If you are in business for yourself, you generally need to make estimated tax payments. Estimated tax is used to pay not only income tax, but other taxes such as self-employment tax and alternative minimum tax.

If you don’t pay enough tax through withholding and estimated tax payments, you may be charged a penalty. You also may be charged a penalty if your estimated tax payments are late, even if you are due a refund when you file your tax return.

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.