If one of the elevators was jammed in the trailing edge up position that must have occurred during the pre-flight freedom of motion check. One would hope that such a failure would be noted during the walk around.
The elevators are also checked during taxi as part of the "free and normal" flight controls check.
On the MD-80 the elevators are checked first by pulling the control column full aft, then by pushing the control column fully nose down until the hydraulic augmentation (Elevator Power) light turns on.
It may be that one of the elevators jammed up during the first part of this procedure -- I don't know how the Elevator Power light is activated to see if this is a possible failure mode.
(The elevators move in response to relative headwind so they cannot be checked for movement prior to taxi).