A friend of mine had this theory, which in a way did make sense. But you know how it goes; if it's not in "The Books" nothing is official. But who doesn't like a good game of theory-craft now and then.
Anyway, her theory was she would have a nocturnal seizure and wake up tired. Due to being tired, this would trigger her partial seizure. Then, a combination of sleep deprivation and fatigue from two seizures, it would trigger another thus resulting in a cluster.
While it was a hoot to hear her say it all (she's very animated, and acts everything out when she talks) - but in a way, to anyone who has seizures, it kind of does make sense.
One day I will tape her talking because it's so funny. She's a hand talker, and makes these funny expressions. I told her I'd make a collage of all of her facial expressions one day and publish it online.