We all have seen the little demonstration of that toy trailer on the treadmill. The one where they take weight off the front and put it on the back and get the huge golly-wobbles, right? But have you ever wondered... why? What is the physics behind the instability. Well, I found an nifty demonstration and discussion of it on a Youtube channel for Airplanes, of all things.
Trent Palmer is a Bush pilot that has a very entertaining series of flying videos, but he is currently grounded by the FAA until his engine manufacturer addresses a potential issue with that line of engines. To keep his video channel going, he has been making some videos explaining things about flying to people, and this last one deals with "ground loops".
I'll let him explain, but a ground loop is when a certain type of aircraft loses control on the ground and 'spins out'. The forces involved are very similar to a RV death wobble if the tongue is too light. When he shows what is going on with a small model plane, it finally clicked in my head, and I understand why we get the wobbles if the tongue is too light.
The video is worth a watch just for that. This link will take you past his introduction and right to the meat of it. I hope this helps someone else understand the dynamics.