There could be many reasons why a horse is stalling, but some of the most common reasons are:
1) Lack of food or water – If your horse is not getting enough food or water, they may become stalled because they are feeling hungry or thirsty.
2) Fear – If a horse is afraid, they may start to stall out because they don't feel safe around people or things.
3) Injury – If your horse has an injury, they may be reluctant to move because it hurts.
4) Rest – If your horse has been working hard all day, they may need some time to rest before they can start moving again.