I believe it occurs when the sun angle is parralel with the top of the open
canopy and the magnification is caused by rays passing through the plexiglass
at almost a zero angle.

I've had it happen twice over the many years. I found that it happens when the
open canopy top is almost parallel to the sun rays, which are reflected off the
bottom surface and are focussed somewhere near the instrument panel. I don't
believe it happens if the sunrays pass through the canopy, since the reflected
rays are significantly lower in strength, having had two surfaces to pass
through. Maybe someone would like to experiment?

Bob