In order to balance when you are standing your centre of gravity should be over your feet. When you are standing and bend forwards your centre of gravity starts to move forward too. Normally, you’d compensate by sticking your bum out behind you.
In this experiment you stand with your back and heels against a wall/door. Drop a coin on the floor in front of you and try to pick it up without falling over.
With the wall/door there, this is impossible, so you fall over.