I'm not sure I'd call it a speculative attack in this case. There's no carry trade going on, because there's no borrowing. There's just merchants using a superior money to do business. Speculation involves taking on high risks for potential gains, typically acting on trends, momentum, and sentiment rather than fundamental analysis. Not saying there's no speculation at play here, but it seems a jump to suggest that it characterizes all or even most of those adopting a sound money standard.