This is the first short story in an anthology from the Yahoo! group called THE PARASITORIUM. It is the second volume pf short horror fiction from the group, called PARASITIC SANDS.
It's the story of a group of three guys who go to the beach for an annual event, watching as instinct takes over, and there's a mass exodus into the ocean, where dead bodies start piling up. Human bodies. People are driven to go to this particular beach, and jump off the dock and into the ocean.
Of course, it's not a story about the annual event, but rather, about the three young men who go to the beach to watch it.
Apparently, the title says it all. There's no mention of what instinct drives these people to the beach, only the carnage and the guys sitting, drinking beer, and talking among themselves.
The end of the story, though... Well, I'll leave that for the reader. The image is actually rather funny.