Pulling someone out backwards is usually a bad idea.
Don't forget that it is WAY more likely to break gears pulling in reverse. Just don't do it.
The stock ujoints break pretty easy but the shafts can break also. Did the shaft itself break or was it the ujoint?
if you are pulling in reverse all the weight loads on the front as mentioned before.
Where your wheels turned? Could bind up a bit or even just add extra stress
Do you have a Auto? The tourque converter can actually preload up quite a bit more power then the engine is putting out. I think I heard something like nearly double in a best case.
Also, do you have an open diff? If so, all those things could play a part.
If you were in reverse, with the load on the front axel, wheel slightly turned...the tourque converter could load up and the sudden transfer of power through the open diff from one side to the other could snap an axle no problem. And it only would have felt like you put it in gearm, gave it a little gas and a snapped like a twig.
Anyway, my point is that it is amazing to me sometimes how easily and how much force can build up on one certain part of a drivetrain and it didn't feel like much was happening. I have seen breaks before that blow your mind that there was enough force to break certain items