Why do military jets sometimes have elevators in a depressed position when parked?
It generally means that the hydraulic actuator (power control unit) driving the surface has an "idle" facility that allows fluid to move internally between the two sides of the actuator piston, or just circulate in the pressure/return lines, and when unpressurized it acts more or less like a hydraulic damper even though the input spool valve is at its "null" (no command) position. 

The result is that when unpressurized it's free to move (although damped) and if gravity wants to pull the surface in one direction it'll do so.  As soon as hydraulic pressure is applied, it'll snap back to whatever position is being called for by the input system.

Sometimes you have PCUs that are "non-idling" and when unpressurized they are hydraulically locked which is only broken when there is a control input to the controlling spool valve.  Such surfaces stay where they are even when the hydraulic pressure has bled off, unless you move the controls to break the hydraulic lock. 

On the CRJs, the elevator PCUs are idling, and the elevators sag when unpressurized.  The aileron PCUs however are non-idling, and the ailerons don't sag when unpressurized. If you try to move an RJ's aileron when its hydraulics are off, it is totally rigid.