Basically....
The reason a forklift has three points of stability instead of four is: the steering axle on a forklift is connected in the center only, to the rest of the forklift.
This is so that the lift truck can drive over a small dip, or driveway and still have all four wheels on the ground.
This also makes turning on a ramp or hill very dangerous because the center of gravity needs only move a few inches to be outside of the "stability triangle" to cause the forklift to turn over.
An unloaded truck is less stable than a partially loaded truck in relation to the location of the center of gravity and the increased possibility of side tipping.