There's a lot of energy in an airstream at 150 mph. A small mechanical slack in the flight controls can find and excite a frequency of oscillation as you experienced. Flutter is a divergent oscillation where air loads oppose one another around an axis of flexibility such as a hinge rod or a wing spar. The frequency is high, and if stress limits are reached by a component the cycling can quickly lead to ultimate stress and breakage. That's when everybody without a parachute perishes or else things get pretty exciting, so I am glad you are ok.

Did you do anything to see if you could alter the flutter? Some things you could have tried might have been speeding or slowing (additional speed might have changed the frequency), trim setting, moving stuff around in the cabin to change CG, or change air densities by changing altitude.

Also, are you sure there was no ice on the tail? Air pockets can be colder than predicted, and flying through cold visible moisture is always a risk. If the tail picked up ice/ frost you might not know it, and it might change the frequency of oscillation enough to make flutter occur. Most likely the slack in the flight controls was there for some time.