1) radioactive isotope of Carbon C-14 or Potassium K-40 decayed in the wrong molecule, in the wrong moment, destroyed DNA of some cell, caused mutation, which resulted in cancer..
2) primary (or secondary) cosmic ray particle (the most likely from the Sun), hit DNA molecule, or surrounding it molecules, transformed them, and damaged DNA like above..
3) free radical reacted with DNA, damaging it..
4) [....]
Some mothers, and fathers, "asked" for it by them self, by drinking alcohol, using cigarettes and narcotics, eating unhealthy and/or carcinogenic food..
Now we have "flood of stupidity", i.e. anti-vaccine movement.. If such people will win, and persuade politicians and spread their stupidity worldwide, we will see more children suffering illnesses almost destroyed in XX century in western countries, and you will go back to XIX century and earlier..