I am assuming you mean a back walkover as opposed to a front? In my experience when I see someone fall backwards in a back walkover there can be a few possible causes. The most common causes I observe are a lack of core strength making the decent uncontrolled and/or poor shoulder and quad flexibility. There are of course other possibilities but without seeing you perform the skill it can be hard to tell.
Can you do a controlled backbend from feet shoulder length apart? Can you kick over from a bridge unassisted? If so then I tell my athletes to think of a back walkover as if it's a reverse handstand. In a handstand uou should stardin a hallowed lunge, arms by ears, reaching forward over your front leg and driving the back. In the back walkover try starting in a lightly arched lunge, arms slightly behind ears, reaching back over the back leg for support while driving your back leg.
Another trick that might help is trying to spot something in front of you as long as possible as you stretch back. This will keep your head neutral and in line with your arms instead of your head going back which will create a shoulder angle, or cause you to get lost and fall.
Hope this helps. Best of luck.
Coach Matt