Having lived in both NYC and SoCal, what they have in common more than crack-smoking warlocks is easy access to MSG-laden Chinese food. My own problem only manifested when I went a couple years eating an admittedly-unhealthy amount of ramen and Chinese (my constant feeling like shit afterward was attributed to me eating like shit...and dehydration), yet they persisted incidentally years later, which makes me wonder if the headaches are a reaction to too much exposure and it's become an allergen or something to me now.

FWIW [takeout] Chinese, kewpie mayonnaise and [Maruchan] ramen are still completely off the table for me, but I can eat a vending machine /kids' size bag of Doritos without issue. The smallest store-bought size will trigger a migraine though. Your wife might be able to indulge a little herself if she's willing to be masochistic in testing how much sets her off.