Hi, I also did not use the Runkai hub. But from my understanding of coaster-type hubs I would not say the driver bearing should be the reason, because even if it is very rough, it does not spin as long as you do not pedal, so even higher restistance should not build up a momentum on the driver to make it spin.
The big bearing behind the driver (which is put into the hub shell on the drive side) would cause momentum on the driver. Imagine this bearing is very rough spinning or even stuck. Then it would necessarily spin the driver and the crank would move... So I would start checking this bearing.
Maybe it also helps to grease the whole system (clutch, thread on driver and so on).
I hope this helps.