why does radiator empty water out of overflow after a drive ?

Insure that over flow tank is at lower limit when engine is cool. Insure radiator has no air in it. Belch it. With engine cool remove radiator cap and idle. Observe for air bubbles being released. Make sure radiator cap is correct one and in good shape. Might want to replace if not sure. Also inspect overflow hose from radiator to over flow. (From cap all the way to bottom of overflow tank). Had one once that had a crack in the line right below the cap of the overflow. This allowed for when radiator heated up the water could get pushed into the overflow but when radiator tried to suck water back into itself upon cooling it would only suck air in thru crack in the line and not the water. Hope this helps