Probly has fleas
Fleas seem like a good reason, they also get those "hot" spots and seem to chew and lick those spots a lot, its also possible they could have a bite or a sticker
Food allergy to a lower quality diet is also a very common reason for that (most common) and also arthritis and things may irritate them. If your food has corn in it and especially high up in the listings it's a great candidate for the cause. You can check out your food on the analysis site, I feed Innova, green bag if you'd like a comparison point. Any switch should be very slow so as not to upset their stomach and usually takes about 3 or 4 weeks for the nasty stuff from the old food to stop affecting them so much and isn't an immediate difference. Your veterinarian can also usually give you a good cream like derma-vet and some other tips to help through a switch if food allergy is the culprit.