The reason that casein or other intolerances may accompany gluten, is the
zonulin link. Zonulin channels are open for intolerant people...they are the
"holes" in leaky gut syndrome. It is thought when these channels are open that things from food get into our bodies and cause havoc.

There also was a study in 1999 that showed that constant use of NSAIDs, block cox-2 in the GI tract. Cox-2 has a beneficial action in the mucosa of the GI tract to keep out things undesirable from our blood stream. So it just may be that many people ACQUIRE gluten intolerance as a consequence of NSAID use.
The escalating occurance of this seems to parallel the availability and easy access of this family of drugs.

So people may not test positive for genetic celiac and still have significant gluten intolerance. The drug industry is working on a drug to close the Zonulin channels. So there is alot of interest there too.