> I think this has the causality reversed.

Originally, yes. The US dollar did become a reserve currency because because of the circumstantially huge negotiating power of the USofA, however, as it stands now, the negotiating power is dependent on the dollar being reserve currency. The US is running on a consistent trade deficit, which is supported by the dollars covering economic growth outside of US.

When self-supporting systems are thrown off-balance, the sign in the feedback loop tends to reverse.