The IRS operates on a "pay as you go" system, meaning they expect taxes for each quarter to be paid based on that quarter's income. Maybe you had one quarter where you had more income than in another?
If your income fluctuates throughout the year, you can choose to "annualize" the income, which will allow for fluctuating estimated tax payments, too. I believe the info in the TurboTax FAQ below will be helpful.
https://ttlc.intuit.com/replies/4242891
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
