South Korea’s KOSPI closed at 6,889.74 at 3:30 p.m. local time on Monday, September 28, down 191.18 points or 2.70% from its pre-holiday close of 7,080.92. The first session after the two-day Chuseok closure forced Seoul to absorb a rise in US Treasury yields, oil above $106 and renewed pressure on technology shares at once.The close was weaker than the intraday reading of 6,936.34 recorded at 10:56 a.m. The index opened at 7,057.86, briefly reached 7,065.90 and fell as low as 6,896.80 before ending below 7,000 for the first time in four sessions. The move followed a volatile summer in which an earlier AI-chip selloff sent the KOSPI down more than 10% in one day.

Samsung and SK Hynix Led the Decline

Samsung Electronics fell 5.43% to 270,000 won, while SK Hynix lost 5.05% to close at 1,768,000 won, according to SBS News closing data. Foreign investors sold a net 3.2403 trillion won of KOSPI shares by the close, far above the 1.008 trillion won recorded earlier in the session. Foreign selling included 1.7275 trillion won of SK Hynix and 1.3757 trillion won of Samsung.

The two companies have carried much of Korea’s AI-hardware rally, making them sensitive to changes in discount rates and expectations for data-centre investment. That concentration was already visible in FinanceFeeds’ assessment of whether the KOSPI was cheap enough after its earlier decline. Monday’s fall also came as investors continued to assess how Micron has narrowed the DRAM revenue-share gap with SK Hynix.

Oil and US Yields Built Up During the Holiday

Brent crude traded above $106 a barrel after US President Donald Trump rejected Iran’s latest proposal for reopening the Strait of Hormuz. The US 10-year Treasury yield reached 5.23% on Friday, its highest level since 2007, while the Korean market was closed.

Higher oil matters directly for South Korea because the country imports most of its energy. Higher Treasury yields create a second pressure point by increasing the discount rate applied to long-duration technology earnings. The combination gave foreign investors an incentive to reduce exposure to the same large semiconductor positions that had led the rally. Concerns about conventional memory supply and pricing are also developing alongside the HBM cycle, including the debate over NAND pricing after a 70% quarterly increase.

The pressure also reached the currency market. The won closed at 1,365.1 per dollar, weakening by 7.6 won from 1,357.5 before the holiday, according to SBS News.

Micron’s Results Are the Next Test

Micron will report fiscal fourth-quarter results on Wednesday, September 30, followed by a conference call at 4:30 p.m. Eastern time. Its comments on HBM demand, conventional DRAM pricing, customer supply agreements and capacity will be read across to Samsung and SK Hynix.

For Korean investors, the immediate question is whether Micron confirms that the memory cycle remains strong enough to offset higher yields and oil. A supportive outlook could stabilize the two index heavyweights. Any evidence of delayed AI infrastructure spending or softer pricing would reinforce Monday’s foreign selling.

Other items to watch this week include Korea’s September trade figures and whether foreign flows turn after the post-holiday adjustment. Samsung and SK Hynix had recently rallied on AI expectations, including the move covered by FinanceFeeds when both stocks advanced on GPT-6 demand expectations. Monday showed how quickly that concentration can work in reverse.