Trade Show Dave is absolutely correct, but here is a less technical answer:

Another way one can look at it, is this:

There are 3 support points, or suspension points, that support the forklift onto the frame of the truck before the truck hits the ground. One is located at each hub of the front drive wheels, and the third, at the pivot box in the center of the rear axle. The 2 rear steer wheels are not connected to the frame of the truck with shocks, struts, etc, There is not a direct connection of the rear wheels onto the frame. But the pivot box is mounted onto the frame, hence the third support point.

Play connect the dots amongst the three support points, voila, you've got yourself a triangle, known as the stability triangle.