I am certainly no expert but you are asking a forum for advise so I will chip in anyway!
I believe it is probably just the way the trailer was made. We have some less severe versions of that crease in our trailer that were there when we purchased it new.
Of course it might be something caused by the dreaded p.o., Marathon tires, too large a tow vehicle, no sway control, too small a tow vehicle, too much load distributing hitch, incorrectly balanced running gear or any of a number of commonly discussed issues
Hopefully someone with a clue will shed some light on this but I really think it was just made that way.....