With your hands behind your back, you are forced to stand erect (except at parade rest). Your shoulders lock your arms, thus making it easier to carry a person.
Watch the clip and you'll see that her hands come in front of her body and she drops like a bag of jello. If her hands would have stayed behind, they could have almost walked her into the van.
Has any reporter anywhere ever asked the Clinton handlers/spokespeople what happened in this video? It is VERY wierd.
Can you imagine the fuss if Trump had collapsed in a similar way?