Because for dogs, this action of humans is an action of preparing to attack, so they will be afraid. Generally speaking, when animals in nature prepare to attack, their hind limbs will slowly squat down, and we humans squat down. In the dog's cognition, it is an action of preparing to attack. They will naturally be scared when they see this action.
Tips:
In addition, dogs are also afraid of people raising their hands. This action is also an attack action for it, so parents should usually Avoid doing these two actions to avoid scaring the dog.
