Has your horse ever done that before?
I'd be very concerned. Did you do anything significantly different from how you normally tack him up? I know its a new saddle, but I'm assuming you've used a western with a front and back cinch on him before?
I'm having a hard time figuring out what the saddle fitting problem would be that would cause a horse to fall, especially if the saddle was not tight and you were not moving quickly.
I'd be cautious about the saddle,and if your horse continues to do it, I'd call the vet and have him checked out.