In many cases they are due to a genetic variant, so they are inherited. If there are other close relatives with polyps you should be carefully screened to see if you also are at risk. In others they are considered to be “spontaneous” meaning we don’t know why a person got them.
There is quite good evidence that tumors may be full of invading killer T cells, but when the T cells are isolated from the tumor they are found to be of low activity. Sometime if you grow the killer T cells in cell culture for a while they regain their activity. It seems that the tumors have learned to secrete an immunosuppressive factor to protect themselves. I am not sure, but I think that can be seen even in the killer cells of the blood.