The canine whipworm (Trichuris vulpis) is a common dog parasite.
The juvenile form of the parasite develops in dirt; it's transmitted by eating dirt (geophagy). It may be particularly common in dog parks 
Some dogs eat dirt.
So do whipworm infested dogs find dirt particularly tasty?
The association between geophagy and worm infection has been studied in humans, but a PubMed query on geophagy and trichuris vulpis in dogs produced no hits (PubMed includes veterinary literature).
I'm sure someone is studying this even as I write this post. I'm hoping for an article within the year.
 I don't know how hard it is to study this. A visit to the local dog park might be the basis for a great high school science experiment.