Which way does the rubber ducky go? The two could be highly related.

Im right handed. If I'm in the tub, I swish the hot water to the back of the tub with more right hand motion than left. This initiates a clockwise motion.

If I shower, the nozzle is aimed more away from the curtain which, in my case, is to my left. This creates clockwise motion, too.

The drain pipes also are factors. Water flow has a non-uniform flow rate through a bend. A bend is, likely, just beneath your tub. This could have some effect on the flow, I think.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.