As it turns out, while this sight is stunning, it's far from rare.
"It's spectacular but not unusual," Dr. George Burgess, Director of the Florida Program for Shark Research, told The Dodo. "Whenever you get an aggregation of fishes like what you're seeing here, there are going to be predators there - even if those predators are normally solitary, like sharks are."
This particular gathering happens once a year, every year. Called the sardine run of South Africa, the frenzy is brought on by a gathering of millions of small fish, migrating Southern African pilchard (Sardinops sagax). Naturally, many fish in one place draw hungry predators, and it's not just sharks. Sea birds, dolphins and porpoises all gather in one enormous, if chaotic, feast. Sometimes, a whale even crashes the party, like this episode in 2011: