It can be triggered by foods, chemicals, and many other substances. Some people even react to such substances as meat proteins, which is fine only if you are a vegetarian. Corticosteroids work very well on EG because it is a type of allergic reaction. Corticosteroids shut down the bodies ability to attack allergens. So you are only treating the symptom.
Corticosteroids also have very bad side effects themselves.
Personally I think that EG is simply a name the medical community has come up with for allergies they can not identify or treat. It is a name for something they do not understand. I truly believe it is as simple as allergy to food or other substance.