When the planet's massive sandworms move, they barrel forward in a straight line. But as Tyson points out, pretty much all legless, worm or snake-like creatures on Earth have to slither in S-shaped lines if they want to move forward.
"Have you ever seen a snake chase you as a straight snake? No!"
That's not mysterious. It's one of the methods snakes use to move forward across terrain that doesn't have things to push sideways off of when they slither. Everyone knows this. Neil knows this. He's just too busy being an ass to remember that he knows it.