Oncology is the study of the development and treatment of cancer. There are multiple ways to answer this question and I will only focus on the basic ones particularly those involving the terms: mutations, epigenetics, oncogenes, proto-oncogenes, and tumor supressor genes.
Cancer is basically what happens when cell replication goes out of control. Cell replication is usually a very controlled process of DNA and organelle synthesis and division. If a problem happens during this process, apoptosis or programmed cell death will occur to ensure that the problematic cell does not multiply. Cancer happens if a problematic cell does not undergo apoptosis or is failed to be killed by guardian or protective cells in the body.
Mutation is basically a change in the certain genes found in the DNA that would otherwise function normally. Oncogenes are the name given to genes that may cause cancer.
A proto-oncogene, is a normal gene, that when mutated and turned on, causes cancer. In contrast, a tumor supressor gene helps in the prevention of cancer, and if mutated to be turned off, will lead to cancer.
There are various oncogenes identified in the human body through scientific research. They are not in themselves bad but because of mutations they can become bad. There are numerous reasons for mutations. Some of them are intrinsic, and congenital while some are epigenetic in nature. Epigenetics simply refers to the change in activity of certain genes through external factors. So technically (although not completely) one can say that a very bad diet can lead to cancer and this is because of epigenetics.
Going to different biological factors that lead to cancer, these include, but are not limited to: an unhealthy diet, an inactive lifestyle, obesity, eating fast food regularly, being born with certain oncogenes that have high links to cancer, having high amounts of radiation, having morbidities like diabetes mellitus type 2, liver cirrhosis, and more.