It's alopecia caused by the root of the guard hair clogging the follicle. Double coated dogs have 1 guard hair (tough and coarse) and up to 4 or 5 downy hairs growing out of a single follicle. This hair grows and sheds in cycles. The guard hair is either firmly rooted, detaching in preparation for shed, or being pushed outward by the growth of a new guard hair underneath, about to shed out.

Basically, if you shave your double coated dog during the root detached stage, the root can fall back down into the follicle and clog it, causing no other hairs to be able to grow out. The shorter the clip, the riskier it is. This can be exacerbated by dirt and dead skin entering the follicle as well. If you do shave your double coated pet, be sure to bathe it biweekly and brush it lightly daily to keep follicles clean and to stimulate growth. Dogs with rapid growth and shed cycles are more likely to encounter this problem, such as huskies, poms, corgis, malamutes, etc. That's also why you can shave your dog several times with no problems but suddenly have it happen.

If the hair loss is permanent however (it doesn't grow back within a year) then you absolutely should go see your vet. The follicle blockage is like hair constipation, it absolutely should not be permanent. If it is, that's a sign of a possibly serious condition, like hypothyroidism, Cushings, severe allergies, etc. You also want to male sure your dog is on a healthy diet that is free of grain and high in meat proteins.

Source: groomer who has taken several skin care seminars from vets.