DNA gel extraction: chemical contaminants
Here are some possible reasons:


Your yield is very low, which you can infer from a low 260 absorbance. Either make sure your restriction digest is efficient or add more substrate to the digest reaction. 
Carbohydrate contamination, which will cause high 230 absorbance. Wash the column one more time with buffer QG (the step is described in the Qiagen gel extraction manual; if you're already doing a wash with buffer QG, do an extra one).
You didn't dry the column enough and some ethanol from the wash remains in the sample. This can also cause high 230 absorbance and your sample with smell of ethanol. Place your column in an empty collection tube and spin it down for a long time (15-20 min). Don't worry, the DNA can withstand it.