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Gearing in the mechanical box thingy that operates the timing of the blades. You probably have a worn down tooth or two. This is the step before "goes totally wacky and out of sync."
You probably don't just turn the wipers off, right? You're a precise kind of guy, so you let the wipers complete a cycle and end in the down position ... so you're starting off at the (roughly) same point in the cycle each time. The teeth are not entirely gone, but worn low enough that when your blades catch some wind it overwhelms the gearing system and hurries it along.
(I have no idea. I'm talking out of my ass. My secretary just had the wiper gear box go on her car and that's how it acted)
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