I think Joe has nailed the problem. It's "oil-canning" which is far worse in aluminum trailers than in fiberglass trailers, but still occurs regularly.
Your rear wall is a "floating wall, hung from the top molding, secured on the sides and bottom by the aluminum molding and the only fastening through the middle is where windows, vents, lights, ladders, etc are screwed "through the FILON and into the frame". Otherwise, the rear wall "just floats there". As the sun heats the rear fiberglass sheet, it expands a bit and that expansion has nowhere to go except to "bulge".
Mine looks very similar and I'd suspect that if you go walking around the back of most trailers, you'll find that they all (for the most part) have some degree of "oil-canning"....
Nothing to worry about, just keep an eye on the edges of the rear wall and make sure that it hasn't slipped out from under the molding. Slipping out from under the molding is not common, only occurs very rarely and nothing you need to "worry about" rather just "be aware of the possibility".... It may happen to one in a thousand, so unless you're that statistical selected one, you'll never see a problem with rear wall separation.
Keep camping, enjoy your trailer and I'd suspect that when the weather cools a bit, the "bulge will diminish in size"....
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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