Geotropism.......roots grow down branches grow up due to responses of chemicals within the cells
A side note.......even though a cascade has branches that hang over the side of the pot, the growth is still up. A tree would look pretty silly with all the branches pointing down, so growing it upside down permanently wouldn't work. You can wrap the pot so the soil doesn't fall out, invert it (either in another pot or between blocks) and let it grow to get the trunk going downward. But, for final tweaking, you'll have to turn it over (say, for a show) to allow the little branches to orient themselves properly. Notice that even on a weeping willow, the leaves are still pointing up even though the branches are dropping.
Or......to keep the growth looking natural you can use good ole wire and weighing
. Much easier to weigh a plant to get it going as a cascade then it is to balance it upside down... trust me...
However, I do know people that keep their established cascades upside down most of the time, righting them only to water. Too much work for me!