@fify-- As far as I know, doctors don't know the exact cause of squamous cell carcinoma on the tongue. The most they can do is identify risk factors, and smoking-- including second-hand smoke-- is a risk factor.
But there are many other risk factors out there, such as a previous HPV infection, exposure to chemicals or cancer causing agents through air, water or food. The possible causes go on and on.
I think that the important thing is identifying as many of these risks as possible after diagnosis of cancer and avoiding them. Quitting smoking and drinking, being very careful about safe sex will help recovery from tongue cancer and it will reduce the likelihood of cancer developing again.
This doesn't mean that someone who doesn't have most of these risk factors will not get tongue cancer. People who don't drink and smoke get cancer all the time. My aunt is one of those people.