Many people believe that weaving is caused by boredom, but while this theory comes close to hitting the mark it does miss it slightly. Boredom itself hasn't been shown to cause weaving, but anxiety and high levels of stress have, particularly as they relate to two specific subjects:
-A general unhappiness with the atmosphere and/or lack of grazing
-Isolation and a lack of companionship.

The most common cause of stall walking and horse weaving comes with separation anxiety. Horses are herd animals and therefore they can become very unhappy if they are isolated from all forms of companionship. Studies have shown that surrounding a horse with others, even if each are in individual stalls, is usually enough to keep walking and weaving at bay. You will know your horse is having separation anxiety if he neighs loudly as he circles or weaves; this neighing is a cry out to other horses.