Think of the aeroelasticity of the front wing sort of like the viscoelacticity of Silly Putty: the higher the force applied to it, the more rigid it becomes.
Around high-speed circuits, the front wings will flex as intended, but they'll maintain more rigidity within the flex, as it were, due to the high aero loads. The low speeds of Monaco, however, make for less aero loading, so the wing is more deformable, which means the vibrations caused by the bumps of the curbs produce effects they wouldn't produce elsewhere.
Source: I'm the idiot who put forth the idea that
the flexible nose on the RB8 was a "mass damper."