It's a thing they do. Killer whales are hella smart. I've read that the current hypothesis is the whales are bright enough to know their eyes are vulnerable for a second to being scratched when they snatch up a seal. So they launch the little fuckers into the air and let them come slamming down to stun or kill them.
Orcas also have a really good pack tactics (pod tactics) and, for animals, a keen sense of cause and effect. There aren't many critters in the animal kingdom bright enough to figure out, "If we all swim under this ice at the correct angle, the wave it causes will wash the seal into the water where we can fuck around with it."