I noticed a trainer had punched a hole in the horses side and it was bleeding a little bit why does that happen sometimes?

What you observed is spur damage, and while it is not an everyday occurrence among conscientious horsemen, it happens often enough in certain training environments that it warrants a direct and honest conversation about what causes it and what it says about the horsemanship involved. A spur that breaks the skin and draws blood has made hard, forceful, repeated, or dragging contact with the horse's side in a way that crosses the line from communication into physical harm, and that outcome is never accidental — it is always the product of either equipment that is too severe for the situation, a leg position that is unstable and out of control, an intentional escalation of force that went too far, or some combination of all three. The mechanics of how spur damage occurs vary. A sharp-roweled spur dragged repeatedly across the same spot during a training session is the most common cause — the rowel catches the skin, and repeated contact in the same area abrades and eventually breaks it. A single hard jab with a pointed or sharp spur applied with significant force can puncture the skin outright. In some cases the horse's thin-skinned nature in certain areas of the barrel makes even moderately applied spurs more damaging than they would be on a thicker-skinned horse, but even accounting for individual variation, visible bleeding from spur contact is a sign that the application exceeded what is appropriate. Skilled, experienced horsemen who use spurs correctly and with a stable leg rarely if ever draw blood, because correct spur use involves controlled, intentional, brief contact — not the kind of extended, forceful, or dragging application that produces skin damage. When you see a horse with spur wounds, you are almost always looking at one of three things: a trainer whose leg position is sufficiently unstable that they cannot control where and how the spur makes contact; a trainer who escalated spur pressure to a degree that reflects frustration or a belief that more force produces better results; or a trainer who is using equipment that is more severe than standard spurs. None of those explanations reflect well on the horsemanship, and it is entirely reasonable to be concerned by what you saw. A horse that is being worked to the point of spur wounds is a horse whose training environment involves more physical pressure than is necessary or appropriate. Good trainers produce compliant, willing, soft horses through feel and timing, not through force — and their horses' sides show it.