Health: Pain and aggression; protein needs.
If your back hurts, you’re more likely to be in a bad mood, and you might snap at people who annoy you. Horses react the same way, according to French research that found a connection between back pain (as assessed by an equine chiropractor) and aggressive behavior. According to the study, of the horses affected by pain, more than 75 percent showed negative reactions or aggression toward humans in one or more tests. Horses with more severe vertebral problems also showed fewer positive reactions toward humans than horses that were less severely affected. The takeaway? Always consider the possibility of pain or discomfort before punishing your horse for negative behavior, especially if that behavior is a sudden change from his normal pattern. (The discomfort could be in any part of his body, not just his back.)
Source: Kentucky Equine Research
An uncharacteristic display of aggression can indicate that a horse is suffering pain.