How does electromagnetic radiation cause cancer?
It is a very complex process that still needs to be understood more..
My take on it is that the energy from the electromagnetic spectrum in particular the part of the spectrum that has high energy causes cancer by making cells in the human body undergo involuntary mitosis (cell replication) which happens naturally but not at the pace EMR causes. Low energy part of the spectrum can also cause cancer but just at a slower rate and also depending on how much one is exposed to.
This then gets out of control and causes tumors and so on.
It also depends on the individual in terms of their DNA and so forth, because not all the cells that undergo mitosis are cancerous.
Hope I helped somewhat :)