Good post. It doesn't make much sense at all. The basic components are the same in most protein powders. If you were allergic to a particular substance, you would experience symptoms from it. People don't just become allergic to things that easily out of the blue, especially when they are constantly exposed to it. Allergies are the result of a hypersensitive immune system. The allergic immune system misidentifies an otherwise innocuous substance as harmful, and then attacks the substance.
This means that the chances of an ADULT suddenly developing an allergy to something that they didn't have a reaction to before is low. In children, there may be the risk of developing an allergy, however, the specific mechanism is largely unknown.