Even when there is no apparent pain, I often find trigger points in muscles. When pressed the horse does show that there is soreness. These tender points are bands of tight tissue within the muscle. When left untreated, pain will eventually show up away from the area of tight tissue.
An example of trigger point pain in people is a headache. When there are trigger points in the neck and trapezius muscles, a tension headache is a common result.
Trigger points are formed by chronic overload of a muscle, and can then cause secondary points to form as a result (sort of satellite trigger points). Other causes are arthritis in joints, fatigue, and trauma. Poor posture can put enough stress on muscles to create trigger points.
In people, carrying a heavy purse or briefcase on the same side all the time can cause enough muscle overload to cause trigger points to form. In horses, I suggest that people alternate the side they mount and dismount on to avoid trigger points in the serratus, long back, rhomboid, trapezius,and posterior pectoral muscles.