Hi, How about this - The jet rushing in to the water creates a 'hole'. This hole is caused by the fact that water is pushed down and away in the direction of the jet. is flowing. The water in the bucket rushes in to fill the hole - a vacuum has been created by the fact that water is being pushed away. When the nozzle is available to plug the hole and respond to the vacuum it is pulled in.
Why does this effect not continue for the entire depth of the bucket? I think this is because the pressure difference is greatest at the air/water interface. Once in the water the energy that creates the hole is expended pulling 'bucket' water in. The flowable bucket water will respond *much* more readily to the vacuum than the 'non-flowing' (solid) hose. I think that is it. A simple vacuum created by pushing water out of the way and by imparting some kinetic energy to it. Chuck. (The elephant is very much like a rope! said the blind man holding it's tail...)
