We think tannins cause migraines because several people have reported that their migraines were completely or substantially eliminated by avoiding tannins in their diet.
One problem with that evidence is that it may be something else than the tannins that is the problem. There may be something else which tends to occur in the same food as tannins which is causing the migraines. We accept this point readily. From a research point of view, it raises an excellent question. From a practical point of view, if avoiding tannins cures your migraines, it matters less.
A second problem is that tannins may cause migraines but not by themselves. We think this is probably the case. We suspect that it is really tannins plus, perhaps, histamines or tyramines that cause the problem. Again, this is an excellent research question. It may or may not matter in practice.
Another problem is the possibility of this being a placebo effect. It is possible that people are avoiding migraines just from attention being paid to their problem rather than because a real cause has been removed. We have not carried out any double blind tests, which are the standard way to test this. But we do have some interesting examples of "accidental blind" experiments. Here are some examples that explain the idea.
On one occasion, the person whose story we told as CaseOne ate about six slices of walnut bread without realising that the walnuts could cause a migraine. She had not had a migraine for several months, but did get one after eating the walnut bread. Only after getting the migraine did she realise that the walnuts could be the cause.
In a story that we haven't yet told, a physician was in the habit of drinking three bottles of apple juice each morning, and then got a daily migraine. When he stopped drinking the apple juice, he stopped getting the migraine. About a week later, he got a migraine from drinking peach juice. It was only after getting the migraine that he read the label and discovered that the "peach juice" contained apple juice as well.
So, for now, we say that "tannins cause migraines." We look forward to scientific research confirming or correcting this statement.