I think this is,in fact,because of temperature extremes. Metal expands and contracts with the warm and cold temperatures. Enamel paint- at least the old lead base paint does not stretch and shrink with the metal and will fail to stick to it. This will create an air pocket between the metal and paint and eventually the paint will flake off. This was a big problem with painted highway billboards several years ago. Does anyone remember seeing highway signs with large chuncks of missing paint in the late winter or early spring?
Also, the old painted signs were often done without using any type of primer. A lot of signs were screened on metal blanks that were even rusty, thus rust spots under the paint.
If your thermometer was hung high in the garage near the roof then over time I think there could have been enough of the expansion and contraction to eventually cause the paint to let loose.