I have no idea what causes this kind of thing.

However, I have had my Cap 21 do someting simular. I never got it down exactly, but once in a while, I'd do a Lomchvack (sp), and hold the controls. Somtimes the plane would fall in to a knife edge spin, but sometimes it would fall in to a scary-fast rotating UPRIGHT spin. (holding full down elevator the whole time here).

I splattered the plane, so I never got to really figure out exactly why it went one way or the other, or how to make it always do one or the other. It also never occurred to me to try an upright spin and then reverse the elevator only to see what would happen.