Velocity is just a ratio. If you double M (which is usually defined incorrectly in the first place) and GDP doesn’t double then V has to get cut in half. It’s just math. But the point is that this doesn’t tell us anything about anything. Doubling the money supply doesn’t necessarily lead to growth. Especially when you define money as reserves. But even if you define money as deposits then the same thing is true. After all, what if I borrow a million dollars and put it in a hole? Or what if I borrow it to buy a business that immediately fails? That’s actually deflationary!
So, the point is that velocity is a silly measure. It doesn’t really tell us the cause since it’s just an ex-post measure of the effects from other things that are happening in the economy.