http://www.sciencedaily.com/releases/2013/08/130814132445.htm
The team found that a family of enzymes, which is known to 'edit' (ie mutate) DNA, was linked to more than half of the cancer types. These enzymes, known as APOBECs, can be activated in response to viral infections. It may be that the resulting signatures are collateral damage on the human genome caused by the enzymes' actions to protect cells from viruses.