Why do I see so many handicapped dorbeetles?
I do not know for certain but one possibility is an internal parasite. 

A number of parasites have been documented to alter the behavior of the hosts to suit the life cycle needs of the parasite.  For example, Toxoplasma is a parasite use rats and cats as hosts. Toxoplasma alters the behavior of the rats so that they no longer respond to the smell of a cat. That makes the rats much more likely to be captured by the cats, which transfers the parasite to the cat from the rat (Berdoy et al. 2000).

This YouTube video shows how a fungal parasite of ants alters the ant behavior to help the fungus reproduce.

My favorite example, which I use in my introductory biology course, is Euhaplorchis californiensis, a parasitic fluke. The parasite lives in marshy habitats, such as Carpinteria Marsh in California, USA. This parasite uses wading birds (herons and egrets), a snail and a fish as it's host.  The adult lives in the gut of the wading birds. Eggs from the adults pass out with the bird feces. Horn snails consume the fecal material and the eggs.  The eggs hatch inside the snail. The larvae actually castrate the snails so that the snails cannot reproduce. Instead, the snails only feed, which provides nutrients for the parasite larvae. The larvae then leave the snail and swim into the marsh, where they are consumed by the California killifish (Fundulus parvipinnis). The parasite works its way from the gut of the fish to the brain, where it releases chemicals that alter the behavior of the killifish. Normally, if a killifish sees a wading bird, it would dive deep. Instead, the parasitized fishes no longer dive. They remain near the surface and swim eratically. This behavior makes it much easier for the wading bird to see and catch the killifish. The parasite is transfered back to the wading bird, and its life cycle begins again. Details of this fascinating life cycle can be found in (Shaw et al. 2010).

Therefore, it is possible that the behavior of the dorbeetles is being influenced by an internal parasite. These dung beetles may consume a parasite when they ingest dung from a certain species. The parasite could alter the behavior of the beetle making them more vulnerable to a particular predator, thereby continuing the next step of the parasite's life cycle. 

Your description of the beetle behavior would fit this hypothesis but I stress that I do not know this. I raise the idea as one possibility among many.

A good not technical book on parasites is Parasite Rex by Carl Zimmer. It covers the biology of parasites, the many ways that parasites can affect their hosts, and the creative scientific minds that uncover the complex life histories of many parasites. (I have no connection to the author or the book.)  The book covers the above examples and more in some detail.