I think there are a number of factors involved. There are a lot of good reasons for eating as local as you can because you become part of the natural biodiversity.
I think some of us can only tolerate so much of the pollution and environment around us and become more sensitive. I'm the only one in my family with "allergies" although most of them have terrible hay fever which is just a grass/pollen allergy. Dust mites and molds are the big things that bother me but I'm also allergic to down and am not supposed to ever keep birds or chickens much less down pillows. I have a few other problems but fortunately no food and very few plant based allergies.
For the most part, in order to react to something you have to be exposed to it first, so you may have been able to tolerate something like shrimp the first few times you had it, but you hit the overload point and your body reacted. Once it reacts, typically it continues to react, and you can have progressively worse reactions.
And then I think genetics come into play in a number of ways, the ability to digest milk and many other things. There are people who are very anti particular things, among them nightshade family plants (tomatoes, potatoes, eggplant among others), or corn or soy. I think there is a lot to be said for their ideas in relation to ethnic groups but not a one-size fits all humans.