As a fellow migraineur, I feel compelled to point out that the quest for triggers and root causes is probably never going to end. The way I see it, the migraine "bucket" slowly fills up, and the final trigger is simply the drop that makes it run over.
I can suggest the research papers by Markus Dahlem for some in depth modern takes on migraine.
Maybe they can find out what empties the bucket.
E.g. meditation, yoga, ...
It's definitely bucket-like for me, and I can attest meditation empties it. Whenever I stop meditating, mental busyness and subconscious anxiety slowly build up. Half hour a day is enough to keep it away. I just keep bringing my attention back to the breath, trying to feel into the physiological need to breathe (which is usually occluded or distorted by mental activity). Whenever I feel I am actively holding to some tension, I allow myself to release it. That's all in terms of instructions, and for me it works wonders. I look at it as the equivalent of flossing for the brain ;)