Wouldn't you agree that it's the pitching moment overcoming the ability of the tail to stabilize the pitch of the airplane.

Pitching moment changes with the generated CL. And is stabilized by the horizontal tail. And the tail does the job through most of the envelope but doesn't at some condition (basically a certain speed).

So we've got a certain speed or condition where the airplane suddenly pitches. Since it didn't pitch just one or two mph before reaching that speed is it just the pitching moment change that does it, or has there been a more radical change somewhere else in the aerodynamics of that model? The CL will have changed very, very little as the speed changed. And the pitching moment will have changed only slightly as well. But the airplane suddenly tucks. Which is a relatively large change in state.

What causes sudden changes in pitch most times? Stalls. Wings stall, and what else can stall all by it's lonesome? The tail? And what would happen aerodynamically when the tail stalled? Well, if the wing was generating any pitching moment there would be the sudden removal of it's stabilizing force from the tail and the model would suddenly go with the pitching moment.

Can the moment slowly build until it suddenly overpowers the stabilizing force. I'm not sure there are any examples of that happening and resulting in massive pitch change. Unless the tail were operating very close to it's stall angle and was pushed into it. And that's a function of the wing's downwash and the tails incidence.

But I'm just musing..............
Never read anything that describes sudden pitching and the reasons. Anybody got a reference?