In case you don't know, though I guess you do, in the Southern Hemisphere wind, and water, circle around a low pressure area in a clockwise direction and in an anti-clockwise direction around a high pressure area.
My shrub must have its own peculial pressure system as when it is say 35mm deep after a shower and I remove the plug it quickly adopts an anti-clockwise rotation. Then when the water is down to about 10mm it stops and adopts a clockwise rotation.
I am pretty sure this is a daily demonstration to me of the cyclonic principles. When the pressure of the water trying to exit through the drainpipe is high there is the anti-clockwise swirl and the pressure difference changes as less water remains.
It puzzled me for ages until I hit on this explanation. If I swirl the water around in a clockwise direction I can upset the natural effect but otherwise this happens every day ... fun eh :-)