There are more than one evolutive way that leads reptiles to lose their limbs, but it's a beneficial trait to the ones that lose it. A good example of it are the amphisbaenas, that aren't snakes, but also don't have their limbs, with one exception, the Bipes.

But in order to do so, they deslocate their jaws, and then they need to do this "yawn" to realocate the bones