I asked this question a long time ago.
Personal theory, as the aircraft yaws say to the left, the fin blanks off the underside of the tailplane, thus its looses some of its downforce causing the nose to pitch down.
Probably why it has such interesting spin modes.
Brilliant for crosswind landing!
Lets face it the Tomahawk is the Mini Metro of the aviation world.
When the last one ends up as a coke can, it will be found jammed in the back of a Cessna 152 seat!