Based upon that account, speed had NOTHING to do with it. It sounds as though the trailer was struck by a vehicle going the other direction. At 50 mph or 80 mph, getting struck head on will cause you to lose control.
Sounds to me like a poor decision was the deciding factor here. I'll even go out on a limb and state that maybe if the pickup drove a little FASTER than he wouldn't have gotten clipped by the semi and wouldn't have crashed. Just saying.
Sorry if you are offended by my comments. I find it rather annoying when misinformation is spread around as fact.
The FACT is that the faster you tow; the worse your mileage is, the smaller your reaction window is, the longer it takes to stop, etc. That doesn't make it less safe though if you have the proper vehicle and driving skills to negotiate those issues.