If you made estimated tax payments, is seems that you did not make large enough estimated tax payments early in the year to avoid underpayment early in the year even though you made payments later in the year to result in a refund. If you incurred a larger amount of your income later in the year you might have to annualize income on Schedule AI of Form 2210 to lower the penalty. For example, if all of your income occurred in Q4, you had no tax withholding and the only estimated payment you made was the Q4 payment, you would have to annualize income on Schedule AI to avoid penalties for Q1 through Q3.
