I received unemploment this ast year, and had 15% of taxes taken out. Why would I still owe taxes on my unemplyment?

We cannot see your tax return.  We do not know what other income you received or whether you had tax withheld from it.  Your tax liability is based on the total amount of income you received and the other variables like your filing status, dependents, credits & deductions, etc. etc.

If you did not pay enough taxes on your unemployment during the year, you may have to pay additional taxes on it when you complete your tax return. The IRS considers unemployment income as taxable income, as do most states.

The difference between what your tax liability is and what you withheld is the amount you owe. Since you stated that you owe taxes, your tax bracket is higher than the 15% withheld on your unemployment. 

TurboTax has a great tool that can help you withhold the correct amount of tax on your income sources. Please see Tax Bracket Calculator for a better estimate on what should be withheld.