Locating external objects includes people. When combined with increased awareness of the position of body parts and someone moves those body parts without adequate warning, that someone is apt to get hit. Spontaneous abilities to hit and kick are apt to hurt or break something or someone.
Sudden, unexpected movements of parts of the body are threatening to everyone. Within ACL 1.8, the information processing speed is so slow that the definition of sudden requires a new definition: less than 20 seconds is sudden.