Is it a total lead change or just a change behind (cross cantering)? If it’s a full on lead change, it may be your weight distribution or pressure on one rein more than the other, causing him to unbalance and switch to make it better.
If he is switching behind or cross cantering, that is probably a physical issue and should be checked by a vet. It could be hocks, stifle or SI injuries.
Another school of thought is that when a rider plays too much with a horse that does auto lead changes, they start to swap. Basically, if your horse can do changes, don’t school them.