The first cause is partial turning on of adjacent pixels when a picture element is turned on. The second cause is turning on a large number of pixels simultaneously thereby forcing a high current through the display's backplane. This current causes the backplane voltage to rise above ground -- possibly far enough to turn off pixels in some areas of the screen that should be on. This second form of ghosting is exacerbated by high display frequencies or resolutions because they tend to require higher currents for shorter amounts of time than slower/lower resolution display modes.
Ghosting is largely absent in active matrix displays.
Shadowing on backlit displays can also be caused by unequal illumination from the display backlight.
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