What worries South East Asia

Emerging markets are especially more sensitive during times of doubt & uncertainty because of their higher risk profiles and hence the large sell-off being initiated is not really a surprise.
The Emerging Asian economy is hugely dependent on exports for its income, and to mention the fact that US is one of the largest market for them is clichéd.
Direct exposure to subprime crisis is not the big concern for these economies because the exposure what they have is negligible compared to other economies. South Korea’s banks, for example have announced a mere $567 million of aggregate exposure to credit derivatives.
Instead the fear is that credit crunch which is already affecting the US growth prospects would soon trickle down to other economies, in turn diminishing the demand for Asian goods drastically. In Thailand for example, export contributes to around 74 percent of its GDP.

The upheaval in currency markets is drawing particular attention because, if it intensifies, it can shatter the expectations of manufacturers and exporters, even in countries such as Japan where companies have been predicting their earnings on conservative foreign exchange forecasts.
The Yen strengthened to as much as Y115.71 against the dollar on Thursday, August 16, its highest level since March. A stronger Yen decreases the value of Japanese exporter’s dollar denominated sales when converted back into local currency.
Incase of Indonesia, the currency has fallen to its lowest level against US Dollar since June last year and hence the central bank has promised to intervene to prevent any further drop in the Rupiah (Indonesian Currency).

In the longer run, any currency advantage for Asian exporters would be offset by the impact on the US economy and lower demand there for their goods. A lot would depend on whether intra-regional trade holds up, but a slowing US economy would affect that as well. And the fact that nearly two-thirds of exports still ends up in the US, European Union and Japan makes it more poignant.