The anecdotal evidence is that 75% of head and neck cancers are directly caused by alcohol and/or tobacco use. New studies are demonstrating that HPV and Epstein Barr virus may also play a role.
For more information:
http://www.oralcancerfoundation.org/facts/alcohol_tobacco.htm

http://www.oralcancerfoundation.org/facts/humanpapillomavirus.htm

Also look under Oral Cancer facts: Risk Factors

I don't know whether or not tobacco played a role in my cancer although alcohol use certainly may have as well as the pot I smoked over the years. I was exposed to a lot of second hand smoke playing in nightclubs until they outlawed it in California. Tobacco use is linked to almost every kind of cancer so who really knows. One thing I do know is that it won't make your health any better.

Neither my head & neck surgeon or either oncologist had any explanations. I think most tend to avoid that to eliminate guilt from the equation. After you are Dx'd what are going to do then anyway? - why waste time kicking yourself? Save it for the fight of your life. Recurrence is an especially bad deal so stopping smoking immediately will have many benefits.