South Korea's stock market fell 5.46% in a day. Here is what broke

The selloff in memory-chip stocks that started on Wall Street on Monday turned into a rout in Seoul on Tuesday.

South Korea's benchmark KOSPI index closed down 348.89 points, or 5.46%, at 6,035.35. At its worst, it was down more than 7% and briefly fell below the 6,000 mark for the first time since late July. A sell-side sidecar, a temporary halt on program sell orders that kicks in when index futures fall sharply, was triggered for the first time in about two months.

Two stocks, most of the damage

The decline was concentrated in the country's two chip giants, which carry an outsized weight in the index. Samsung Electronics fell roughly 7% to 8% and SK hynix roughly 9% to 11% during the session, depending on the snapshot.

Foreign investors drove much of the selling. According to Chosun Biz, foreigners pulled 3.72 trillion won out of SK hynix and Samsung Electronics in a single day, about 82% of their total net selling in the market. They have now been net sellers for six straight sessions.

The KOSPI broke above 7,000 in early October. It has since fallen about 13.8% in five sessions and dropped below its 200-day moving average, a widely watched gauge of the longer-term trend.

What broke

Analysts and local media pointed to several pressures arriving at once.

• A Chinese competitor. ChangXin Memory Technologies, or CXMT, said it plans to release a next-generation DDR5 product for servers. CXMT has yet to reach mass production, and its yields, performance and customer certification remain unverified, but the plan revived worries about future memory supply and pricing.
• Doubts about AI spending. Comments from chip industry executives about the capital required for AI investment, and the returns it will generate, weighed on sentiment. The Seoul Economic Daily reported that Broadcom Chief Executive Hock Tan, speaking in Singapore on Monday, said the current memory shortage could look different a year from now.
• The end of buybacks. Samsung Electronics' 15 trillion won share buyback program ended on October 6, and SK hynix's 40 trillion won program is due to wrap up this month, removing a steady source of demand for the two stocks.
• Rates and oil. High US Treasury yields and Brent crude near its highest level in four weeks added to the pressure on risk assets.

The link to Wall Street

The move started in the US. On Monday, Micron Technology fell about 3.9% and other memory names dropped after the CXMT news, helping push the Nasdaq Composite down 0.44%. Korea's chipmakers then extended the move, and US memory and semiconductor stocks fell again in early premarket trading on Tuesday. Nasdaq 100 futures were down about 0.4% to 0.6% before the open.

Samsung, SK hynix and Micron are the world's three largest memory makers. Memory has been one of the hottest corners of the AI trade this year, because the high-bandwidth chips used alongside AI processors have been in short supply. That makes the group especially sensitive to any hint that supply could catch up or that spending could slow.

Why it matters beyond Korea

The KOSPI has become one of the world's most direct proxies for the AI hardware cycle, because two memory makers make up such a large share of it. A drop of this size is a signal that investors are asking harder questions about how long the spending boom lasts, not just about one company's results.

Some local analysts noted that the market was falling on risks that were already known, and that thin trading had reduced resistance to selling. Whether that proves right will depend on fresh evidence.

What to watch

That evidence arrives quickly. ASML reports on Wednesday and TSMC on Thursday, both widely read as gauges of demand for AI chips. US September inflation data on Wednesday will also shape the rate outlook that has weighed on high-valued technology stocks.