I think he probably became overheated on the trip home... not at the kennel.
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.