How did Kim crash her car?

Could someone please explain this because one minute she's driving down the road and then the next an airbag randomly explodes in her face. It was shown pretty clearly that her car drove off the side of the road and ran straight into a rock. Surely there would have been at least some sort of shaking as her vehicle drove into the ditch. It was just completely random, but maybe I'm missing something here. It just doesn't seem like the writers would fuck up something like this, seeing as how they've made a pretty good show thus far. Don't get me wrong, I like the scene, but it just doesn't make sense to me. Any explanations?