A diaphragm with no tension is inherently unstable. Precisely inbetween the two stators, equal force pulls it in both directions since the distance to the two stators is the same making it quasi-stable in that position. Due to manufacturing tolerances, it will not be precisely in the middle, and even if it were even the slightest deflection would move it closer to one stator. When it is closer to one, it is attracted to it. Even if the stator is neutrally charged, electrostatic induction will cause the electric field between the neutral stator and negatively charged diaphragm to induce a positive charge in the surface of the stator thus creating an attraction to the diaphragm.
Tension provides an opposite force to the instability so that when the diaphragm is slightly deflected in one direction, the forces of the tension are greater than the electrostatic attraction to the stator and the diaphragm is pulled back to an equilibrium point. So with enough tension, the quasi-stable system turns into a system with a wide region of true stability.