This could be because the shaft assembly is binding some. I would take the inside assembly apart and soak the moving parts in vinegar overnight or several hours to remove any deposits. Then reassemble, dont overtighten the nut. ... make sure the prop. spins freely. Should be a little bit of up-down play in the magnet/shaft. Then try it again. If the nut is too tight the entire assembly can spin off when the motor is started. SAme thing if it is binding do to calcium build up or related.
If it still does the same thing, contact ecotech and send it in. They are great with service and fast turnaround.
I think this may have been one of the issues reported when the Gen 2 first came out. But I think it usually happened shortly after installation. So yours may just need a vinegar bath and cleaning. But you could always ask ecotech in their forum first to see if they suggest to try the same. If so, Make sure you take note of how it goes back together.