There can be any number of reasons, but the most likely is that you didn't claim enough allowances on your Form W-4.
Not claiming enough allowances results in too little withholding from your paycheck, and eventually owing money once you file your tax return.
Please see the TurboTax Help Video: Why Do I Owe on My Tax Return? for information.
We're unable to see your return in this forum, so we can only speculate.
