My horse started weaving it's pretty bad so quickly what is the cause stress?

Weaving — the rhythmic side-to-side swaying of the head, neck, and sometimes the entire forehand that a horse performs repetitively, usually at the stall door — is a stereotypy that once established becomes neurologically self-reinforcing and very difficult to eliminate. The fact that it has developed quickly and severely is actually useful diagnostic information — a rapid and pronounced onset typically points toward a specific identifiable stressor rather than a slow accumulation of management deficits, which means there may be a specific cause to identify and address. Stress is the correct general category but worth being more specific about. The most common triggers for rapid-onset weaving are sudden changes in the horse's social environment — the loss of a herd companion, a move to a new facility, separation from a specific bonded horse, or a significant change in the management routine. Horses are social animals with genuine attachment bonds, and the abrupt removal of a social relationship or a stable routine can produce the anxiety that stereotypies develop to manage. If your horse's weaving began around the time of any such change, that connection is likely causal rather than coincidental. Confinement is the environmental factor most consistently associated with weaving development. A horse that was previously turned out for significant portions of the day and has recently been moved to a management situation involving more stall time is a horse whose behavioral needs are suddenly significantly less met. Similarly, a horse whose exercise level has dropped significantly may develop weaving as an outlet for energy and stimulation that work previously provided. The management response that has the most consistent positive effect on weaving is increasing the horse's social contact and environmental stimulation. A stall that allows the horse to see, hear, and ideally touch other horses reduces the social isolation component that drives much weaving. A mirror placed at the horse's eye level in the stall has been shown to reduce weaving frequency in some horses by providing a visual social stimulus, though the effect varies between individuals. Increased turnout, more forage availability, and restoration of whatever social arrangement existed before the weaving began are the practical first steps. Weaving that has been present for weeks or months is more neurologically established than weaving of very recent onset, and the horse whose weaving began recently has a better chance of meaningful improvement with management changes. Act quickly and address the most likely cause first.