Why does my horse's saddle slip backward?

I ride in a crosby prix de nations saddles with no extra pads, just a saddle pad and a standing martingale. My horse is a 16-17 hand warmblood. The saddle looks fine when i get on and i am sure the girth is tight, my trainer checks. When i get off, the saddle is way to far backward, which is bad for his back. Why does the saddle keep slipping back?