It is a combination of aerodynamic and mechanical instability. The quickest fix for our models is to make sure your linkages are tight, servos adequate, and surfaces strong and properly hinged.

Action and Reaction:

Flutter happens when some force (usually an aerodynamic force) impacts part of the control surface system - it could be a change in attitude, implementation of aileron/elevator/rudder, a wind gust, or possibly even something like vortex shedding (spiraling air that breaks off opposing edges of a surface creating an oscillating instability). Whatever force is present is the action.

Every action has a reaction. When the above action is beyond the limits of the surface/servo/linkage, something will bend, break, or give. If it doesn't break, it will snap back like a rubber band (servo regains control when force reduces, push rod flexes and reflexes back, etc). It snaps back with a force that is about equal to the initial force, however the aero force is probably gone or moved. Now you have a large reactive force with nothing to balance it out, and the process continues until it finds some kind of balance. Hopefully it oscillates out, but often something breaks.

Flutter has a lot to do with the bending and resonant properties of the materials of the control surface and linkage, and even the wing and airplane frame. So the cause of flutter is usually inadequate structure (including loose parts) for whatever situation the airplane is in. The aerodynamic force sets off the flutter, but is not really the cause. Airplanes should be able to fly within their operating envelope and not experience flutter.