Sideways moving centre of gravity
Now that the centre of gravity has moved towards the rear of the forklift truck, it does not have to move very far in a sideways direction before it crosses the red line(s) shown here of the stability triangle, causing the truck to overturn in that direction.
It should be noted that if the centre of gravity had not moved towards the rear it would have much further to travel sideways before it crossed the red line. The fact is that taking a full load to maximum height on full rear tilt puts the truck into its least stable position sideways.
It is for this reason that trainees are instructed to remove either all or most of the tilt before elevating the forks prior to stacking.