Perhaps he did overheat on the way home, because of agitation, which is one of the causes of hyperthermia. A beagle would probably just have gotten hot, but if this was a very driven shepherd, the excitement may have sent his body temp through the roof.
FWIW, I found a number of references to Malignant Hyperthermia, a killer of black labs. This is associated with a reaction to some anesthetics.
This story doesn't tell us much at all. After reading it I thought, maybe the dog hung around the house for several days exposed, and of course got dehydrated and "hot"! So maybe it had nothing to do with hyperthermia, just over-exposure. But again, the story doesn't make it clear at all, just as with how he escaped.