While widespread use as transaction medium and the size and liquidity of underlying financial markets also sustain a currency’s use as store of value, reserve currency status has additional, potentially more volatile determinants, including expectations about future monetary and fiscal policy as well as strategic considerations epitomized in China’s "dollar trap”.
Further factors are geopolitical influence and the quality of the political, economic and judicial system. A historic example for the vagaries of reserve currency status is the fate of the US dollar and pound sterling in the interwar years.