From page two:
"21 August, a 47 year old vendor who witnessed the entire event, recounted to reporters, that during the time of the incident, the traffic was heavily congested. She noticed a black, four-door pickup truck waiting in the jam when a group of teenagers on a motorcycle approached and began knocking on the pickup's window, trying to force the driver out of the vehicle.
The driver of the pickup truck attempted drive away and flee, but one of the teenagers got off the motorcycle, armed with a metal rod, and opened the truck's door. The teenager then stabbed the driver, who was a foreigner, leading to a struggle as the driver fought back in self-defense. The event, captured on video, left the vendor frightened. She immediately called for nearby residents to intervene and quickly contacted the police."
Maybe the Thai guy wanted to attack the foreigner with a weapon, so he did.