This term was coined by a DogsBite.org forum administrator aptly named, Landshark. It’s meaning stems from a famous Saturday Night Live skit, which parodied the movie Jaws. In the skit, Land Shark is a cunning urban predator (voiced by Chevy Chase):
The Land Shark is considered the cleverest of all sharks. Unlike the great white shark, which tends to inhabit the waters and harbors of recreational beach areas, the Land Shark may strike at any place, any time. It is capable of disguising its voice, and generally preys on young, single women. – Wikipedia
A pit bull attack is indicative of a Land Shark attack as it strikes without warning or provocation leaving its victim dead or with permanent disfiguring injury. Pit bulls even strike when an unsuspecting person opens up his door, as featured in many of the SNL skits. The reverse is true as well, when an unsuspecting person knocks on a door.