Ok here go's in laymens terms:
Any wing can flutter. To get flutter you must have a few things:
1. A wing that is not stiff enough. The wing or surface (vertical or horozontal tail) will help dampen the flutter (oscillatory motion): Now this is a tradeoff between weight and stiffness. A wing build out of solid wood would be very stiff...a 3D airplane is built Extremely light but with less stiffness...a lot of 3d airplanes can not be flown very quickly without flutter. (Full size also)

2. Something to "drive" the surface...now this could be unattached flow on the back of the wing....but more than likely the aileron drove the wing into flutter...now when this happens the flow starts the trailing edge of the surface one direction then the other...if there is any slop in the controls the aileron will start moving back and forth. this will put a "back and forth" force on the wing trailing edge. The small force will build up (It's kinda like pushing your little sister in a swing a small force at the right time will make the sister go higher and higher)....

So, make sure all the surfaces are tight...hinges are tight...linkage is tight...etc....and don't fly the plane faster than it was designed for which is slow for a 3d plane..faster for a sport model and as fast as it will go for a pylon racer.