The most common neurological disorder preceding / associated with epilepsy is?

Brain injury: Scarring of brain tissue can result in epilepsy. Birth injury, brain infections, head trauma, drug toxicity, environmental pollutants, can all result in scarring of brain neurons that then develop into epileptic foci. Tumors and strokes scar the brain and can result in epilepsy even when the tumor or stroke resolves.

Depends upon age: The "most common" cause of epilepsy depends upon the age of the person. In the first years of life, birth injury, genetic causes, etc. Middle years of life--head trauma. Later in life, strokes and dementia. There is no age group for which brain tumor is the most common cause of new-onset epilepsy.