This is an allergic reaction---odd though it sounds--thus the use of corticosteroids to treat it. For this reason, they often return because the corticosteroids don't "fix" the allergies long-term.
The allergic reaction can be due to insects, materials (such as whatever your saddle pad is made out of), foods, plants, etc. Since most people and animals that are allergic to 1 thing are allergic to several things, it can also be a combination. This was discussed at the 2006 AAEP meeting and one of the comments made by the presenting veterinarians was that doing food trials and removing any feedstuffs that the horse is allergic to from the diet can stop the nodules from developing. Basically you need to figure out what is triggering the allergic reaction and get rid of it or minimize it.
You might want to consider having allergy testing done on your horse to help you pick out the triggers so that you can remove them.
Licensed Veterinary Technician