I'll throw my hat in the ring. Here's a scenario:

In a 3D "wall" with the strong pitch up from level to vertical (90 deg
pitch up in ~.1 sec), there's a very large pitch rate. On the
Funtana90 with a big prop, this large pitch up causes the prop to
produce a gyroscopic force that yaws the airplane to the right. Since
the airplane at this instant is seeing a very high angle of attack,
it's in the "post-stall" regime. But with the strong yaw, the left
wing is not as far into post stall as the right wing. In other words,
the right wing is seeing a higher post-stall angle of attack. This
setup in right/left wing aero is somewhat similar to the entry into a
snap roll, and that's what I think is happening -- a slight snap roll
(~1/8 roll) to the right.

I'll emphasize that I think this sequence only occurs in a very rapid
pitch-up with, say, a 50-deg of elevator deflection on a 50%+ elevator
control surface like the Funtana90 has. This is the kind of move that
can nearly blow off wings. Also, none of this works unless the
gyroscopic force is high, which requires a big prop, high RPM and also
a high-speed entry to get the high pitch rate. So all of this is a
pretty extreme maneuver and airplane setup.

Michael