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The powder is either bridging or getting caught somewhere, and the return movement of the drum shakes it loose.
Two possibilities jump to mind:
Powder is bridging in the drop tube. That can occur with either "loggy" powders like the original 4831 or with a drop tube with too small a neck section (or both). Most measures come with at least two drop tube inserts. It can be a mistake to just screw in the small one and suppose that it can be used for all cartridges and powders.
Less likely is the possibility that the rotor drum isn't turning all the way or over-turning (or not correctly aligned). That would leave a small shelf or lip between the rotor and the drum housing - a place where a few kernels of powder might collect on the downstroke. Then, when the upstroke starts, the powder gets swept off.
Remedies are simple: Use the correct drop tube insert and check that the drum isn't misaligned with the housing. Check the rotor operation by eye with the measure empty and the drop tube removed.
Finally, a powder baffle can sometimes make real improvements in the flow of the powder and consistency of the measure. Insert one in the powder storage tube.
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