Why would a quail dig in a dirt bed then flip dirt on itself?

It's called a dirt bath. They dig until the dirt is very fine and then flip it onto themselves until it gets under their feathers. The very fine dirt smothers any lice or mites the birds might have. Dirt is also very good to clean with as well to absorb excess oil. Desert Birds generally do this because of the scarcity of water. Matter fact it works with human hair as well, just might not smell as good as soap :P