Even the same KINDS of cancer behave differently in different people. Some are caused by environmental exposure (smoking, exposure to chemical toxins, etc), others are genetically based.
This makes the disease VERY difficult to treat in many cases. We've made tremendous progress with some cancers. When I first became an oncology nurse at Roswell, the overall five year survival rate for all cancers was around 25%. Now it is more than 60%. We've made great strides with certain types of cancer, some not so much. Unfortunately, the type of cancer Jim Kelly has is very difficult to treat, especially when it recurs (squamous cell cancers).
But to answer your question, the most common causes for squamous cell cancer of the mouth are tobacco use (smoking or chewing), alone or in combination with a lot of alcohol, and/or the presence of a particular virus that can lead to this type of cancer down the road.
Regardless of the cause, this particular type of cancer is a real booger, especially when it recurs.