Dynamicity inside a stationary water drop

Based on the similar movement of inhomogeneities on soap bubbles, I'd say the flakes' movement is caused by air currents around the drop and perhaps thermal convection.

_{Source: https://youtu.be/LM3p3X92mWI}

Those are flakes of lampblack. Their wettability in water is unequal around their periphery. This allows surface tension forces to apply a netforce in some random direction on the flake and force it to scoot around, until the wettability difference goes away as the water begins to wet out the previously poorly-wetting spots.