Three million people had to be moved to safety zones in southern China! why?

More than three million people had to be moved to safety zones in southern China as Typhoon Mangkhut moved northward and continued to wreak havoc across the region.
 Landslides were recorded in Guangdong, China's most populous province, late afternoon Sunday, killing four people before heading west into neighboring Guangxi province around midnight.

The decision to evacuate towns and cities in southern China came as Hong Kong was left reeling by ferocious winds of up to 173 kilometers per hour (107 miles per hour) and gusts of up to 223 kph (138 mph).