I don't know a thing about fluid dynamics. This is just a layman's understanding. But the idea is that as water rushes in from the hull breach, it then sloshes

I don't know a thing about fluid dynamics. This is just a layman's understanding. But the idea is that as water rushes in from the hull breach, it then sloshes from side to side. It's rhythmic based on the size of the boat and how long the water takes to slosh back and forth.