Answer
To stand, your center of mass must be closer to your feet.
Work Step by Step
When you rise from a chair, initially your CM s not over your point of contact with the floor (i.e., your feet), which acts as a pivot point. Your weight exerts a torque about that point that tends to rotate you back into a sitting position.
Therefore, to stand up, you instinctively first lean forward to move your CM over your feet.
