I agree with David. As long as you do not obsess over trying to find out where this may have come from to a point that you are letting this question affect your daily life and possibly your good health. Trying to find the best possible answer to this is fine. But, you must also be able to eventually accept that there are some questions (and this very well is one of them) that you will need to accept as never being completly answered. In saying this. I believe that researching this question "where did it come from" can be beneficial in your understanding of the pathology, treatment and recovery of cancer. and understanding eventually too that there is still so much to learn and still not known.
Now I will share with you some of the information I have received from medical professionals in regards to the same question. My husbands symptoms and initial diagnosis are similar to your Mothers. It started with what appeared to be a line of leucoplakia on the left side of his tounge. the biopsy confirmed SCC. That was 01/05. His life style up to that was nothing that would have put him at risk for mouth cancer. (see below signature) Now let me go back further. Around 1999/2000 my husband was treated for postive skin cancer ( he is very fair and has lived a large portion of his life outside without sunblock.) on the RT side of his face. SCC (We did not remeber what kind until the oral cancer and asked for the records) They excised the area with clear margins. the Dr. also treated the rest of his face for any precancer cells with a cream that my husband applied for two weeks. It supposedly killed the bad cells and didn't harm to good. after that he was sent on his way with a word to return if he ever had anymore unusal spots. No one made the correlation at first. Not even when we questioned the connection with one of his oncology Dr.'s who said," no, totally different". Well actually thier not totally different. And it is a fact that skin SCC w/o treatment can spread to organs. SO the question really still remains.( the tounge is a muscle not an oragan) there is no definitive proof one way or the other and we will never know and are ok with that now. This experience has made me remember the prayer the Alcoholics use. The final sentence I believe goes. " And to accept the things I cannot change". I hope you can soon find the "calm within the storm" that we have found on our journey for answers. It will come. And I wish you and your Mother all the best.