Well it's physics. The wind shield on top of the truck means there is an area of dead air on top of the trailer. A lack of wind to blow off the snow means the snow builds up.
A melt freeze cycle creates a sheet of ice on top of the snow, which expands to cover the remaining snow.
Cross winds will remove some of the snow on the back of the trailer. When enough supporting snow is removed the ice sheet cracks and breaks off.