Why a lift truck can tip over forwards?

Lift trucks (fork trucks, stackers, etc.) are rated for the maximum weight it can safely care at various heights of the forks (blades). The higher the forks the less stable the truck becomes, especially if trying to lift a heavy load, or turning too sharply at speed. The same goes for reach trucks, with or without side shift. So, if the load is too heavy, the rear of the truck will lift off the ground.