No one knows exactly what causes all cancers. Proof of this can be found in the simple example of sperm count in the last 10 years. It's important to note that cancer is NOT a single disease. Instead, it is defined from the tissues it comes from, and is a diverse cluster of diseases.
Cancer changes the DNA of a cell, making it grow and divide unstopably.
The remaining 10 percent of cancers are caused by several factors, not all of which are understood.