There may be a few points to clarify: there is no such thing as an Asian currency. Each country in Asia has its own currency: Korean Won, Singapore dollar, Thai Baht, etc. The value of these currencies is determined in the foreign exchange markets, based on supply and demand of these currencies. Some countries, like Singapore, Hong Kong or China peg their currency to another currency, such as the US$. This means that the central bank of that country has to interfere in the foreign exchange market to buy and sell its domestic currency to maintain a constant value against the US$.

Most of the Asian countries are export economies. With the world economy, in particular the US, recovering from the recession, the demand for goods produced in these export economies begin to increase. As a result, there will be a higher demand for these Asian currencies, because the exporters are paid in US$ but have to pay their workers in domestic currency. This excess demand for domestic currency, drives up the value of the currency against the US$ and, if the value of the currency is fixed, the central bank has to continue to buy US$ and sell the domestic currency to maintain the currency peg to the US$. This will increase the domestic money supply, causing an increase of the inflation rate. To avoid such inflation, the central banks will have to let the value of the domestic currency appreciate.

You are right, if other Asian countries let their currency appreciate against the US$, there is no direct need for China to do the same. However, there is considerable political pressure on China from other countries to also let the Yuan appreciate, because keeping the value of the Yuan low against other countries, gives China an ‘unfair’ advantage for their exports: with a lower Yuan, the Chinese exports are cheaper compared to countries that let their currency appreciate.