Why are mentally retarded people prone to seizure disorders?

Wiring: Because their brain cortex has areas which are not "wired correctly" in a sense. Some of the circuits can feed back on themselves, causing hyperrhythmic bursts of electrical activity which spread, and which disrupt normal circuit functions.

It may be genetic: Children and adults with special needs and developmental delay may have an underlying genetic reason that ties together learning, language, and at times epilepsy. We don't always find a reason for this, however, frustrating families and their doctors.

Brain damage: For some, their retardation is due to brain damage. It is the damage that makes them prone to seizures.