It is not at all unusual for a horse to pull back and keel over rather than jump forward. If they do it once, they are more prone to do it again.
Many things can get this behavior started and pain has nothing to do with any of them. Since you saddled him just before this happened, it is my guess that you may a girthed him up a little too tightly. That is one of the easiest ways I know to get a horse to set back and/or keel over. It is also the way most cinchy horse get that way. Cinchy horses are very prone to set back and keel over if they are girthed up a little to tight all at once. I have a couple of old trail riding horses that I am pretty careful not to tighten them up until they have stood around for a while or walked around until they are gradually girthed up tightly enough. I have 'inherited' every cinchy horse I have ever had and not one that I have saddled and trained from day one have ever done this.
Horses will set back for a number of reasons. I would have to know more about this horse and his background to even venture a guess on his setting back if it is not connected with a tight cinch. But again, quite a few horses that set back will keel over. If they are not choking themselves, I will usually let them stay down until they decide to get up. I have known several that would do this just to be let loose.
Always be sure you tie this horse as high or higher than his withers. They will seldom hurt their necks if they are tied high. If a horse is tied lower than his withers, they can tear or even rupture the ligament in front of their withers. This is know as have a 'pulled down' neck. It is a permanent disfigurement and really ruins a saddle horse. They have this permanent dip in front of their withers and cannot lift their heads and necks very high.
If I have a horse that has set back and particularly one that has fallen over, I will always tie that horse with a 14 foot lead-rope that is run through my tie ring and then tied off to the side in an out-of-the-way place that can be reached and released if the horse goes down and is choking or in real trouble.
Chances are real good that he will do it again.