It can be several things. One Farleyv described. It can also be allergies - not necessarily food-related, but potentially environmental.
When Chumley (our FIV+ kitty) started doing this, we got rid of every chemical in the house, and started shopping for all of our cleaners (dish soap, laundry soap, carpet cleaner, etc.) at the health food store. Though of course, now our supermarket carries Seventh Generation stuff. Chemical-free, dye-free, scent-free.
We already had the house packed with various air cleaners (filters) because of my allergies.
I believe your kitty is already on a grain free diet? Of course, the protein source is a potential. But you need to check ingredients for brewer's yeast. That's a common ingredient that's a known allergan.
When these "environmental" and food-changes were made to Chumley's diet and he didn't improve, we searched
http://www.ahvma.org for a DVM trained in alternative medicines and nutrition. We found one we could get to.
She is trained in Chinese Medicine. As Farleyv's vet said, he was pulling out his fur because he's too hot. Based on where the fur was missing (the same places your kitty is missing fur), and the condition of his lips and tongue (tongue is very important to diagnosis in chinese medicine), he suffered from an excess of heat. He had an extreme yin-deficiency and had an extreme spleen Qi deficiency. She prescribed herbs for him. I couldn't get the first ones down him. She changed the prescription. The second one was not capsules, but large tablets. I was able to cut them in half and put them in a pill pocket that he'd eat. The pill pocket had wheat gluten, but we decided it was a small enough amount that the benefit of getting the pill in him was worth it.
Within a month it was clear - he was overgrooming less, and MUCH less manic about it. Within a few months, his hair was all grown back in on his leg, and clearly coming all back in on his abdomen (takes longer to grow back there). We did up his dose to three pills a day, and within six months, his coat was
amazing. Thick, full, glossy, SO SOFT, and any grooming he did was clearly out of enjoyment and normal cat motivated reasons, not in the least bit stressed or manic.

Chumley also had rodent ulcers that we could not see - it was just a thickening and redness on the inside of his lips, and his tongue was thicker than it should be and red on the edge. Not something I could see until she pointed it out.
If you do not have access to a holistic vet, if you want to try the same herbal formula that worked for Chumley, I do not think it can hurt given the treatment addresses the energy imbalance creating it. It is called Prosperous Farmer, and you want to find one manufactured by Kan Herbals because of the quality of the herbs and consistency of the formula. It is much cheaper if not purchased from Kan Herbals but a distributor, though which one is cheaper depends on shipping costs and sale prices, which vary.