A bike with a vertical ("90 degree") fork would be very unstable. But not because there is no rake, it would be unstable because there is no trail.
Agreed, I guess I should have explicitly said 0 offset.
Atl, read about the mechanical trail that is source of dissonance and since I'm more of doer than teacher, maybe that will explain it more clearly. It also clearly explains why the OP would experience what he did. Everyone is using the formula posted above, saying there is more trail, and then applying that to known research and common knowledge of mechanical trail which are 2 different things. Mechanical trail being what my crude image shows above.