Hot/Cold back-pressure?

With our old water heater removed, my wife decided to do a cold wash in the nearby washing machine. Result: water gushed backwards out of the open hot-water pipe going into the wall above where the heater used to be. Im still trying to figure out how running cold water can cause this back-pressure in what I thought was a disconnected hot water system, unless it is a function of the mixer inside the washing machine. Ideas?