Many factors can play a part in hip dislocation after replacement. For example, which side of your hip is the scar located? Any position you get in that can push the hip in that direction has the potential to cause a dislocation.
When you are on your hands and knees, you have your body weight against that hip. If you twist or angle your body against the hip, injury can occur. Your weight and bone density are also important factors. Being overweight means that much more pressure through the hip. Having osteoporosis (brittle bones) or decreased bone density makes it harder for the bone to grow around the new implant and hold it in place. Bone or muscle weakness can also lead to injury.
Most patients are given positioning precautions for the first 12 weeks post-op. In theory at 12 months you should be free to assume any position possible. In practice, sometimes our theories (and hips) don't hold up.