Here's what I found - A brain tumor is an abnormal mass of tissue in which some cells grow and multiply uncontrollably, apparently unregulated by the mechanisms that control normal cells. The growth of a tumor takes up space within the skull and interferes with normal brain activity. A tumor can cause damage by increasing pressure in the brain, by shifting the brain or pushing against the skull, and by invading and damaging nerves and healthy brain tissue. The location of a brain tumor influences the type of symptoms that occur. This is because different functions are controlled by different parts of the brain. Brain tumors rarely metastasize (spread) to other parts of the body outside of the central nervous system (CNS). The CNS includes the brain and spinal cord.
no one really knows for sure but traumatic injury to the head could be one of the very common causes also genes, if that persons family has a long history of brain tumors it is very likely it could get passed down from generation to generation