I suspect there are multiple reasons for the DNA changes. One that comes to mind is the number of times the lungs have to repair themselves due to the tar and other contaminants. The hot smoke might even contribute to the tissue damage. Cancer is uncontrollable cell growth. The cell/body's ability to control a certain type of cell division has been lost.
So, my thinking is, in the same way that you lose quality as you make a copy of a copy in a copy machine the same happens to the cell's DNA. The more a cell has to divide the less the DNA can remain without errors. DNA can tolerate a number of errors but it can eventually lead to cancer. My guess is that not one issues causes the DNA changes but and array of them given the number of substances a cigarette has.
So, my thinking is, in the same way that you lose quality as you make a copy of a copy in a copy machine the same happens to the cell's DNA. The more a cell has to divide the less the DNA can remain without errors. DNA can tolerate a number of errors but it can eventually lead to cancer. My guess is that not one issues causes the DNA changes but and array of them given the number of substances a cigarette has.