It does not fall into "GMO" category nor are they concerned about the pesticides. It is the actual protein in corn that can cause allergic reaction in a small number of population.
Yes, polypeptides are primarily responsible for allergic reactions, the point was that allergy to maize is significantly less common than other crops and some other kind of labeling motive seemed probable.
I don't believe I've seen an acceptable threshold listed for maize allergen by any regulator, and most don't have established thresholds even for the more common allergies because peoples level of sensitivity is highly variable. One allergic person might tolerate 40ppm, while the next could go into anaphylactic shock and die at 5ppm. Many regulators have simply fallen back on "labeling must be true" and if you test your product or otherwise know it contains even a ppb trace of the allergen, it has to be listed if its one of the top X allergens that regulator recognizes as significant.