On alot of 27% and larger airplanes... why is it that alot of the time the elevator and ailerons sag down when the pilot turns the power switch off?

When power is supplied to the servos , they park themselves as the trim on the radio has them set. When power is turned off the servos go to a relaxed postion, and gravity pulls down on the control sufaces , thus the sag , when power is turned off.

I've seen airplanes with two servos per wing and two servos to operate the elevator and when the power switch is turned off both the main wing and horizontal tail feathers both of their conrol surfaces sag down (like you were flipping on a flaperon switch) at the exact same time. This can not be caused by gravity because there is no way that the control surfaces weigh enough to push these high powered servos down. Any suggestions would be appreciated.

Yes its gravity!

Insanemoondoggie is telling you the truth. Thats how digital servos work. Without power there is very little resistance in the servo, so even the slightest weight will move them.

True.

If these were standard servos, gravity would not have enough power to overcome the servo's resistance.

But Coreless BB servos have very little resistance when not powered and are very easily moved when the power is off