Why do I owe taxes for 2009 when I made almost $20,000 less than last year and received a refund for 20008?

In 2008, I was employed full-time and my taxable income was $40,266. In 2009, the bulk of my income was through unemployment. I also had about $3000 in self-employment. My taxable income was $22,613. Would the unemployment or self-employment have anything to do with it? I followed the 1040 instructions and double checked my work. I don't see how this is possible.