You can gain speed quickly when heading downhill on a scooter with the weight of two people. The key is to avoid building up too much speed in the first place. Additionally, the driver may have experienced target fixation, which is common among inexperienced riders. There’s also the possibility that he hit gravel, dirt, oil, or other obstacles. In Thailand, they aren’t exactly known for practicing proper maintenance, which can include issues like worn tires, worn-out brake pads, and not bleeding the brakes properly. However, in the photo, the path does not seem very steep, and the turn isn’t particularly sharp.