a long time now, and have read the Celiac boards for years.

I can't recall finding out the "why" it is higher in some people than others.
It just "is".

If you Google it there are many hits, including the drug company who has a compound in trials.

Here is an interesting Wiki, with illustrations:
http://en.wikipedia.org/wiki/Tight_junction

I think PD along with other CNS neuro problems, may have a connection to gluten intolerance.
Here is the Gluten file:
http://jccglutenfree.googlepages.com/

It is a huge collection of data, including neuro issues and very helpful.
Going gluten free may be very helpful for some people here.