I have dealt with splay legs quite a bit....have one here with all four legs splayed. There can be several possible causes.
One possibilityis trauma. But, this usually affects only one leg and if it does involve more than one leg and involves the hip and/or pelvic area, the degree of splay is not symmetrical.
A genetic error can also cause splay legs. This can be from just the random genetic "tick" or could come from a compressed gene pool....inbreeding.
Housing on a slippery surface is also a possibilty especially if it were in the first few weeks of life as the bones were very soft.
And something that most people do not know about....Metabolic Bone Disease...aka MBD. This is a condition that we see very often in wildlife. MBD is nearly always a nutritional issue. Has to do with the absorbing of nutrients.....especially calcium. We can many times trace the root cause of this issue to a Vitamin D deficiency. It is generally recognized that a lack of Vitamin D comes from a poor quality diet and/or no exposure to direct sunlight....not thru a window or screen....direct sunlight. There is some evidence I have seen in some papers that lead to the benefit of sunlight being made thru the eyes....and not the skin. Symptoms of MBD can vary but it is much like osteoporosis in humans. One of the major presentations is bone loss, deformities in the bones and joints and moderate to severe pain. Other presentations may include stunted growth, a course fur, lack of energy and in extreme cases it may include organ failure and seizures. Calcium is one of the key elements to prevent MBD....and why I cringe when I hear people trying to totally eliminate calcium from a rabbit's diet especially when the reasoning for this is flawed. Many commercially available foods lack the fortifications that include supplements that would prevent MBD.....and this includes some pellets that are considered "top of the line".
I would suggest having an orthopedic veterinarian specialist take a look and try to determine the cause of the splays. If it is mechanical (like no balls on the femur or no sockets in the hips or lack of tendons/ligaments) there might not be much that can be practically done. If the tendons or ligaments are there but weak, it might be possible for a surgical repair of some type of rehab....and hydro-therapy has been successfully used in rabbits. If the bones are curved due to being soft, it is many times possible to perform a procedure known as "hobbling" in which the legs are braced under pressure in an attempt to reform the bones. I would also perform a full CBC to check the blood and function of the vital organs. Make sure the diet does have proper fortifications to provide proper components. If you feed an all veggie and/or hay diet, you might need to look into a Vitamin D supplement. As with anything, there can be too much of a good thing and this applies to Vit D/Calcium/Phosphorus supplements....it all has to be in a controlled amount. But these components are essential in absorbing nutrients.
Splay legs also add some husbandry issues. Most splays have some problems with friction injuries and skin infections on the abdominal area. They are also incapable of cleaning their ears on their own. And especially as they age and add weight, the un-natural position and lack of mobility can lead to abnormal pressure on the vital organs. The lack of mobility can make them suspect to excessive fluid in the chest cavity.....aka as Congestic Heart Failure or Pulmonary Edema.
But if you willing to make the effort, there is certainly the possibility of some degree of correction to splays....and we have some that have lived many happy years with just minimal housing modifications.