Many bird species stick very close to their preferred habitat. In fact, certain species can ONLY be seen in very specific habitat. And herons are known for standing like a statue in shallow water, waiting for a tadpole, fish, or other aquatic meal to swim by, at which time their lightning-fast reflexes snatch it out of the water.
This heron was exhibiting some of the same stalking behavior in this field. And after a few minutes, it snapped at some sort of rodent on the ground. For some reason the Saturday Night Live skit involving the 'Land Shark' came to mind. :)
Apparently land feeding by Great Blue Heron is not totally uncommon. Places with lots of rodents and few predators provide a good option for an opportunistic heron. For what its worth, this heron was stalking the grounds of McClellan Ranch Park in Cupertino, CA.