Why do horses tend to try and change leads in a rundown?

A horse that attempts to change leads during a rundown is one of the more frustrating problems in reining because it disrupts the straightness and pace that a correct rundown requires, and it tends to get worse under the pressure of competition when the horse's adrenaline is elevated and his attention is divided between what the rider is asking and what his own instincts are telling him to do. Understanding why it happens is essential to fixing it correctly, because a lead change attempt in the rundown is never random — it always has a cause, and that cause determines the solution. The most common reason is anticipation of the stop and subsequent rollback or spin. A horse that has run a significant number of patterns begins to associate the rundown with what comes at the end of it, and if the maneuver following the rundown consistently goes in one direction, the horse starts preparing for that direction before he has been asked. That preparation often manifests as a lead change attempt — the horse swaps to the lead that sets him up for the anticipated turn before the rider has given any cue to do so. The fix is to vary what follows the rundown in training — sometimes stop and stand, sometimes stop and roll back left, sometimes right, sometimes stop and back — until the horse stops predicting and starts listening. Balance and collection issues in the rundown are the second major cause. A horse that is strung out, heavy on the forehand, or drifting to one side during the rundown is a horse that is struggling to maintain the correct lead comfortably at speed. When a horse becomes unbalanced on a lead at a hard gallop, swapping leads is sometimes the horse's attempt to find a more comfortable way to carry himself. That swap is the symptom of an underlying balance and collection problem that needs to be addressed through correct gymnastic work — circles, transitions, and straight-line gallop work that builds the horse's ability to carry himself correctly at speed on both leads. Straightness is the third factor. A rundown that curves even slightly to one side puts the horse on a bent track, and a horse traveling on a bent track at speed will naturally want to swap to the lead that corresponds to the direction of the bend. Riding the rundown on a genuinely straight line, using both legs to keep the horse's body straight rather than allowing drift in either direction, removes one of the most common physical prompts for the swap. Gate sourness and barn anxiety contribute in certain horses as well — a horse that is mentally focused on the gate rather than on the rider's cues is organizing his body around where he wants to go, and a lead change in that context is the physical expression of a mental departure that started long before the swap occurred.