we can't see your return. so as a guess, your withholding was less than what was needed to meet your tax liability. if this is the case, we can't say for sure why that occurred, but it could be that you claimed too many exemptions on your W-4. if you need a better reason, you can upgrade for a fee to live support, where they can review your return
This can depend on multiple things. If your too young to qualify for EIC, and you claimed more than 1 throughout the year, its very likely you didnt pay enough taxes out of your check each week. Or if your income was to high and you had no deductions you could end up having to pay in at the end of the year.
Hope this helps