If you are using gimbaled weapons and have your targeting set to trailing this will happen. The reticule is showing you where the shot will land, but as your target changes speed and direction your weapons are moving to track it so this point will be continually changing. As you get closer to the target this effect will become more pronounced. As Error says, if your target uses chaff then your weapons won't lock and the reticule will dance around all over the place.