Doesn't Like The Rider Mounting
If the saddle doesn't fit properly it will be uncomfortable on your horse, but it will be most uncomfortable when someone sits on the saddle. All of those pressure points, that are hardly noticeable when the saddle has no weight on it, suddenly dig into the horse and hurt him. The heavier the rider the more they will feel this pain. When the rider is about to get on, the horse will associate this with pain and will do his or her best to stop it from happening. He may try to escape you when you are trying to get on, or he may kick out at you or try and bite you.
Use The Perfect Fit Saddle Fitting Kit to check if the saddle fits. If it doesn't, be sure you get one that does and don't ride him in the old saddle until you have. Using the kit, check the saddles's fit at least every month, or more often if there are changes to your horse's diet or exercise.
When you are certain that the saddle fits well, if the behaviour is maintained, it could be that your horse remembers the pain that he was in previously, or, he is still in pain from the damage done by that saddle. If this is the case, you must have his back checked by an equine physiotherapist, chiropractor or osteopath. In fact, if a horse exhibits any unpleasant behaviour, the first thing to do is have him checked out by one of these professionals.
Sometimes it can take a while for a horse to trust that the saddle isn't hurting him, so be patient with him if he takes a while to become relaxed in the process. Sometimes the upset that the pain causes him can create an unbalance of the healthy gut flora. This, in itself, is enough to cause your horse to be tense and exhibit undesirable behaviour. This unbalance can last a long time, so it is always worth giving a supplement of probiotics to a difficult horse.