The problem of the cause of this effect has been featured in Scientific American magazine, and has been solved by a computer model of a typical bathroom. The spray from the shower-head drives a horizontal vortex. This vortex has a low-pressure zone in the centre, which sucks the curtain.
One solution to the problem is to use shower curtains with magnets strung along the bottom that afix to the side of a metal bathtub.
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