Yes, I think the slippage of the tire is what caused the tear at the base of the valve, so I have now glued the bead on one side of the tire, to the rim. I had a relatively safe ride home.
For what it's worth: no, the valve is and has always been perfectly perpendicular to the rim. But that doesn't mean it couldn't have been torn. I do believe, at this point, that the most likely scenario for my tube breakdown modes was, in fact, slippage of the tire.

EDIT: by "slippage of the tire", in this context I meant "slippage inside the rim", not on the road.