Usually Great Blue Herons move very slowly in the water and are very still before they pounce on their prey. In this case, though, this bird spotted a fish, ran wildly, then dove in for the capture.
The GBH had just caught and swallowed a fish, and the Pelican had hurried over to try to steal the fish. The GBH was having none of it, and the Pelican was in quick retreat mode. :)