He's using a running rig - one swivel is running on the main line, the other is a stopper with a leader to the hook.
I'm assuming your using this on the beach?
To fix your problem, shorten the leader between the running swivel and the sinker, you shouldn't need 20cm for that bit. Try tying it down short to 5cms or less and they shouldn't wrap around each other.
Another way to sort it out is to remove the sinker leader altogether and just run the main line through the eye of the sinker. You may need to use a bigger stopper swivel or crimp the sinker eye a little, or you could use a snap swivel on the sinker instead of a leader.