Gluten intolerance and other allergies are very real, sometimes fatal and have nothing to do with being highly strung as you say Sparky.
Among JWs as for many outside the cult, individuals hear of problems others have with certain foods and because of their reputation (like gluten) they avoid them as if this information will benefit them also. I remember a stupid sister who ate all and any medicines since she reasoned that medicine was there to help you. It is this sort of thinking which drives superstitious people to collect food fads and prohibitions. When I was a JW I did find JWs were most susceptible to this type of thinking but through lack of of data it would be hard to compare the frequencies with the general population.