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.

Don't let this fool you, this reptile is a lizard, not a snake. This is the Anguis fragilis, one of the many lizards that lost their limbs. We know that this is a lizard, and not a snake, because of some traits like the eyelids and the capability to do autotomy, and also the tongue is not forked.

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