It's because the ridge acts as a vortex generator, but you get a small controlled continuous one. Without you can get the chimney moving to the side one way, which tends to increase the attachment of the airflow on that side, so you get lift (in this case laterally) so the chimney keeps moving. Until finally the force is too big for the aerodynamics to overcome, it stops moving sideways, the lift collapses, the force goes and then the chimney goes back to the middle. On the way of course the process reverses, and you get lift going the other way - hence the oscillation.
Vertical ridges would do just as well - except they have to be on the side as the wind sees it. So much easier to put a spiral.
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