One important factor that no one here has mentioned is that the dog could have been the cause of the accident. Was the dog secured? Evidently not since he is missing. Did the dog distract the driver?
Also, was this accident at the beginning of the trip or near the end? If was at the beginning of the trip the tow vehicle and the trailer may not have been connected properly?
If the accident was at the end of the trip could it have been driver fatigue?
Was there something inside of the trailer that may not have been secured and there was a weight shift?
What it all boils down too is, be safe, check everything, and know how to use your equipment.