EDIT : solved, thanks :)
Depending on the shape of the object the lift on the front and rear halves isn't the same and this would cause it to pitch up/down if it wasn't spinning. As I understand it the spinning converts this pitch effect into a roll effect.
I believe this effect is worse for flat plate like objects and less for frissbees like shapes that are designed to balance the lift in the fore/aft plane.... but I'm no expert!
Perhaps read up on the physics of the frissbee?
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