If you ever feel a behavior from your horse or dog is due to something deeper verses them being spoiled or needing a behavior correction... go with your gut... because to many times now I have seen it stem from the gut or pain elsewhere.
After two different vet's and many months of one of my clients and good friends trying to figure out why her dog wouldn't hardly even eat the finest chicken, lamb, etc... after being told by one vet it's just a spoiled dog... she found her answer and the dog had surgery...
Seen this in a horse too. Behavior begins to escalate and change, I recommended a diet for gut health, horse calmed down.
Different horse, similar behavior, was either ulcers or kissing spine from my observation at the time of appointment. Owner got x-rays and turned out to be kissing spine.
Often times when we are faced with an observation from our animals, begin to look at them as a whole. Finding the fine line between a behavior that needs correction vs them trying to communicate to us the only way they know how, is the difference between progress or an accident waiting to happen.