One of the trailers was constructed exactly as you discribed above and the other (if I were to hazard a quess ...) was a H16 trailer that had a longer trailer tongue installed for a H18, but no other modifications.
I think the location of the mast step was the critical factor ... combined w/ how aluminum fails .....
What I found interesting is that both trailers seemed "bouncy" before they failed ... both had the same type of failures, at exactly the same mid-point on the trailer tongue .... so (I theorize ...) if you stay away from "simple" ratios on were your mast step is mounted such as 1/2, 1/3, 1/4, etc to the overall length ... you shouldn't generate any harmonics/resonents frequencies in the trailer tongue. ( ... you can use a calculator and check for a non-repeating "odd" number ... and use that to determine were exactly to mount your mast step ... a few inches either way can make all the difference ...)
Note, that I didn't origonally assemble the trailers, so I don't know why they mounted the mast steps were they did ... (easy math ???) ... only fix them .... and could only surmize at the actual cause of the failure .... and I haven't seen any fail like that since .... but .... I also haven't seen anymore "Trailex's" assembled like those two either ....
H-18 mag/ #9458
Fleet 54 Div 11