I'd guess it's because when the plane is being held by hand then your hands are dampening most of the vibrations that are causing the foaming. If it's restrained by a tail hook or whatever then the model is free to vibrate in all directions and if the engine revs happen to coincide with the natural frequency of the fuselage you'll really get some vibrations. This is usually worst on a profile fuselage where there's not much rigidity left to right.