The first Igloo cooler I had did that, so I bought another one. It does it too. And, it did it first in the SUMMER, so I don't think a cold garage had anything to do with it.
The first MLT cooler flexed back and forth until it cracked along the high edge of the "hump" formed by the heat. The second has not cracked...YET.
So, even though coolers hold in the heat GREAT, I am dropping them for a very heavily insulated keggle MLT. Hopefully, the present cooler MLT will hold together long enough for me to finish the keggle one.