It seems that nearby infantry units end up touching the main rotor of the WY-55 when they're near it, and they seem to move it around a bit.
I tested this by doing the following:
- Create a WY-55, name it heli
- Create a trigger, condition 'isTouchingGround heli', activation 'hint "Touching Ground"', deactivation 'hint "Not touching ground"'
- Walk around the helicopter and see what the hints tell you.