My horse started running out at the fences once in a while what am I doing wrong?

The assumption embedded in your question — that running out is something you are doing wrong — is worth examining carefully, because while rider error is certainly a common contributor to running out, the behavior has enough possible causes that attributing it automatically to the rider before investigating all of them risks applying the wrong correction. Running out occasionally rather than consistently also tells you something useful — a horse that runs out at every fence has a different situation than one that is generally willing but runs out in specific circumstances. Rider position and approach errors are the most common contributor when a generally willing horse begins running out occasionally, and honest self-assessment with video is the most reliable way to identify them. The most frequent rider contributions to running out are an approach line that angles toward the side of the runout, and an indirect rein on the side of the runout that has opened or softened in a way that creates an invitation to drift through that gap. Watch video of the approach from directly behind the horse — the approach line, the horse's straightness, and the rider's rein and leg positioning are all clearly visible from this angle. The horse's physical state is the second area to evaluate. A horse that runs out more frequently as sessions progress, or that has started running out after a period of willing jumping, has potentially developed a physical reason to avoid the effort or discomfort of jumping. Veterinary evaluation of the back and hind limbs is warranted if the runouts have developed recently in a horse that was previously confident. The fence itself and the questions it presents to the horse deserve consideration. Occasional runouts at a specific type of fence are informative about where the horse's confidence and training gaps lie. Addressing the training question that the fence presents — through gymnastics, approach work, and systematic building of confidence at that specific question — is more productive than simply re-presenting the same fence more firmly. The preventive riding correction involves specifically closing the leg and the outside rein through the final three strides of every approach rather than waiting until the horse has already begun to drift. The outside rein and outside leg together create the channel that keeps the horse straight — applying them proactively in every approach produces a horse that encounters a consistent channel rather than an occasional opening that the runout exploits.