What causes colorectal cancer, and who is more likely at risk of developing it?

Can be genetiic. Can run in the family but there is an increased incidence with age. That is why a colonoscopy is advocated over the age of 50.
Multiple causes. There are multiple causes to colorectal cancer. It is a combination between genetics, and what your colon & rectum are exposed to over the course of many decades. Clearly the most at-risk patients are those with a strong family history and a documented history of gene mutations, namely something called fap and hnpcc. Take a look at some of my other posts on reducing your risk.