Subsequent to this post, I discovered another trigger: Potatoes, of all things. I had eaten a hamburger and fries on Friday night. Most of the hamburger bun I had peeled away, but I had unwittingly devoured all of the fries. Putting 2 and 2 together after some subsequent incidents, it suddenly all made sense. White bread + potatoes is a sure combination to generate a headache.

Mind you, it's not all bread: I've had pitas that were white as clouds and nice European table bread in Italian restaurants with crust so thick you have to tear it with your hands... that stuff doesn't seem to phase me. Makes me wonder if it's the yeast.