Cancers evolve. They lose the ability to protect DNA, so they can get mutations very fast. Each cell can get different mutations. Some of these will give the cancer an advantage, like being able to escape the immune system.
Some of them enable the cancer cells to leave the tumour an travel to other sites in the body via the blood.
Cancers spread over the body when the ‘metastasise’. This means that the cancer no longer has barriers around it to stop it going from one place in the body to another. The cancer has a big army with it of blood vessels and the ability to dodge through cell walls. It is big enough to now go through these areas on its own an spread.