alt Hacker NewsIt starts with mutations (sometimes accelerated by mutagens (smoke, alcohol, etc) or inflammation (viruses, infections, etc) or just chance (things like asbestos up the division rate by constant physical damage and thus up the probability or an error in copying).
But there is much more to it. This is a nice paper for an overview: Hallmarks of Cancer (tng) [0]. It (among others) adds the very important and for years underestimated role of the immune system to the original 2000 paper.
Would sports also increase the rate of division? Bodybuilding for example you intentionally make micro tears in muscles to get them to repair and grow (lay person, apologies for the mistakes)