The root cause of cancer is cellular stress. One cope mechanism with cellular stress is the cells fusing. On occasion a blood and a somatic cell will fuse. A blood cell is an organism on its own, disconnected from the electric network of the somatic cells which controls the growth and shaping into functional tissues and organs. When the programming of the fused cell combines in a way that it wants to grow and divide while disconnected from the body control network, you get a cancer.

https://m.youtube.com/watch?v=EWNjw3pg9sA

Pretty much anything can cause cellular stress, including the normal operation of worn cells in an old body. Cataloguing the strength of the stressors is still useful.