I know it's up to the owner/driver to distribute weight in a safe way, but I'm surprised that the wrecked trailer had the axle so close to center. I own 3 trailers (1 camper, 2 open cargo), and the axles are all about 1/4 to 2/5 trailer length from the rear, so when empty they're all front-heavy.
The location of the axle isn't important, as long as the weight is distributed properly YOU MUST ALLWAYS HAVE AT LEAST 10% OF THE TOTAL GROSS WEIGHT OF THE TRAILER ON THE TONGUE. The manufacture designs the trailer so that it has the proper tongue weight empty. Then it is up to the owner to load the trailer to maintain that balance. As you pointed out the are all designed "tongue heavy" so they will tow properly unloaded. Several years ago a Friend ask me to tow his trailer to the lake where he had a leased site to leave it. I had already picked it up and towed it home for him so I knew it towed fine. So I backed my truck up to it and he lowered it down on the ball and fastened everything up. We drove 25 to 35 mph to the interstate ramp no problem, as I accelerated down the ramp it started to sway so I pulled over, got out and checked inside to see what they had loaded, everything look fine. By this time he had pulled in behind me to see what was the matter. I explained something was wrong and ask him what he had done, he said nothing the only thing he had done beside clean was to load the cement blocks in the rear compartment that he was going to set the trailer on. We unloaded twenty cement blocks from the trailer into my truck and the trailer towed perfect. The lesson I learned was never let anyone else hitch my rig unless I was carefully watching. Recommended tongue weight is 10 to 15% if that weight exceeds or hitch rating then you need an equalizing hitch.