To add to this: we can't determine the exact reason why cancers were caused in the great majority of cases. However, there are cases and certain types of cancer where we can determine with great confidence that a certain cause was at its origin.

For example, in cervical cancer, usually caused by a virus called Human Papilloma Virus, that same virus leaves a kind of hallmark when viewed under the microscope called a koilocyte. AFAIK, detecting cancer of squamous cells in the cervix together with koilocytes OR the actual isolation of the virus is pathognomonic for HPV-caused cervical cancer.

For people with familial cancer syndromes, like Familial Adenomatous Polyposis (FAP), if a person comes in at an early age ie. 25 years old, and we see on colonoscopy a colon that is littered with growths, and after biopsy one of those growths comes back as malignant, we can tell with a high degree of certainty that the familial cancer syndrome (the heritable gene defect) was at the cause of the cancer.

Another type is a mucosa associated lymphoid lymophoma of the stomach. If we find a bacterium called Helicobacter pylori in the stomach, together with a tumor we determine to be a MALT lymphoma, and especially if that tumour regresses in antibiotic therapy, we can determine with high certainty that H. pylori caused that exact tumour, IIRC.

Actinic keratosis is a skin condition that's caused by chronic excess UV-exposure (usually the Sun). It can lead to the development of skin cancer. If skin cancer arises from actinic keratosis, we can say with a high degree of certainty that UV light caused it.

There are many more examples: liver cancer in a patient with an alcoholic cirrhotic liver; esophageal cancer on the part closest to the esophagus in know stomach acid reflux, etc etc…

these may not be ‘caused by this definite exact cause’ but ‘caused by this to a high degree of certainty’.

There's bound to be more examples in genetic cancer syndromes (such as Li-Fraumeni, Multiple Endocrine Neoplasia) and infectious cancer syndromes.