Why is there white build up on the bottom of fermenting container?

Here is the pic

Quoting Charlie Papazian's, "The Homebrewer's Companion":

"I have sometimes noticed with curiosity a dusting of yeast adhering to one or perhaps all sides of a bottle of bottle-conditioned beer. It was a total mystery to me until I came across a summary of some research Belgian brewers had undertaken to unravel this aberrant behavior. They found that yeast is electrically charged the same as glass and is usually repelled. Sometimes if the yeast is deficient in certain nutrients, it will take on a different charge and become attracted to the sides of the bottle."  Endquote

I periodically get this in the carboy during primary fermentation.  If it seems to me to be a little too excessive, I hold the neck of the carboy with one hand and the other under the bottom...  I give it a quick twirl left then right without shaking it up too much.  Most of the dust will come loose and then settle to the bottom.  I usually only need to do this once, seldom twice during fermentation.  You could probably do the same with a bottle and it will settle to the bottom. 

Fermentation is over and the yeast is falling out.