if your frame is sloped 15 degrees from vertical, then your chains are too close to the base of your sled because you are using a heavy router and the center of gravity is too high. Lift the chains and maybe space the motors out as well.
I found this picture on another post on the forum today:
you want your sled to hang like #4 so it matches the slope of your frame. The problem is when the sled is at the top of he board, the chains are not holding the sled from the top/front of the sled, but from the side, so if you are using a long 1/4" bit, the router is back away from the cutting surface and the sled can tilt back. By spacing the chains out, they pull the sled back forward into the work piece.