A normal cell becomes cancerous when a highly pleomorphic "cell wall deficient" bacteria is able to get inside of a normal cell.
Once inside the cell this bacteria blocks the formation of ATP molecules inside the cell. The cell then reverts to fermentation to create ATP molecules and the cell is then defined to be "cancerous."
While the fermentation causes a cell to be defined to be "cancerous," the fermentation is caused by a reaction of the cell to the "cell wall deficient" bacteria.
Note that DNA damage does NOT cause a cell to become cancerous, rather DNA damage is the result of the DNA of the bacteria interacting with the DNA of the cell. For more information about how a normal cell becomes cancerous see this article: