If anyone gets these and is in the dark as to a cause, I have found a very strong correlation with excessive caffeine intake. I have been wrestling a caffeine addiction under control, and have found that these always occur on the days when I relapse and have many cups of coffee or energy drinks. It isn't bidirectional-- lots of caffeine doesn't always cause the migraine-- but the migraines always happen only when I've had a lot of caffeine. Whereas when I'm on the wagon and consuming none, or normal amounts, I never have them anymore.

I'm not sure if it's the caffeine in and of itself or downstream of something like higher blood pressure, but the correlation is extremely strong.