South Korea’s world-beating stock rally came to a sudden halt as the benchmark tumbled on Friday, underscoring the vulnerability of a market dominated by a pair of chipmakers.

The day started on a buoyant note, with the Kospi Index hitting the 8,000 milestone and triggering celebration in trading rooms. But those gains quickly evaporated as foreign investors extended their selling streak, largely cashing out of technology shares. Samsung Electronics Co. closed down 8.6% and SK Hynix Inc. fell 7.7%, pushing the index 6.1% lower.