A pad is a pad and although they do vary in types, thicknesses, etc., they do sometimes move a little, but shouldn't be squirting out to the back. I have a strong suspicion that it is the fit of your saddle.
First, I would assess the shape of your horse's back. How is his weight? Does he have those wide quarterhorse shoulders? Are his withers pronounced or flat? Is his back long or short? Straight or slightly swaybacked? Run your fingers over his spine and see if he flinches at all to check for sensitivity. Rest the saddle on his back. You should be able to get 2-3 fingers vertically in the space between his withers and the arch of the saddle resting over them. Slide your hand between the saddle and the horse and check for anything rough, pressing in, or irregular about the two surfaces that are meeting each other. Check the channel running the full length of the saddle down the horse's spine and make sure that it is well off the spine. The saddle should end 2-3 inches before the stifle hair marking the beginning of the hip. Especially check where the saddle sits on the withers. It shouldn't be pinching and worse than the rest of the saddle. Is the saddle seat level...not high in front or back. If the saddle is pinching in the withers, the saddle will sit high in front and you will be able to get more than two or three fingers measurement between the top of the withers and the horn. If the saddle is too wide for the horse, this same measurement with the fingers will be reduced. Then do the girth up and see if the saddle feels like the girth tightening has raised the back of the saddle. It should sit square and tighten down squarely. I suspect that the withers of this saddle may be too narrow for your horse and when the saddle is tightened down, things tend to squirt out the back. When you put the saddle on, always put it on the horse a little forward, then slide it down the withers until you get the feeling that it has slid into place. When you do up the girth, you should be able to get your hand under it still. Doing the girth up too tight is not going to fix this problem, just aggravate it. You should be able to mount up with a secure enough feeling that the saddle is not going to slip around on you and that is tight enough. Checking the pad for compression memory, checking the horse's back and saddle for irregularities, as well as sweat marks when you remove it, will all give you clues to any problems. The sweat should be present when you remove the saddle...its where there are spots that are dry, that should raise suspicion. Check these things out and imagine that its your back wearing this saddle and try to be sensitive and observant. Good luck.
