Reflex epilepsy (also known stimulus-sensitive, triggered, or sensory-evoked epileptic seizure) is a reaction to internal and/or external triggers. The external triggers can be sensory, electrical or biochemical and these triggers can act in conjunction with internal factors such as hormones, hydration, sleep deprivation, intoxication, etc. to cause seizures.
For instance, someone with musicogenic epilepsy might have seizures after hearing a Mozart sonata (external trigger) which then triggers emotional memories (internal trigger). Understandably, this kind of epilepsy is incredibly difficult to predict due to the number of variables involved.