Recent large study published in nature may shift scientist believes about the genomic origin of cancer cells..please read the valuable article attached.
lot of these variations occur on environmental pressure, as for the skin cells where UV induce variations but not all variations survive due to DNA repair functions until cells are stressed and all process tired and she down. another paper on the same topic is : https://science.sciencemag.org/content/364/6444/eaaw0726.long.