Approximately 15,000 people were inside the SEG Plaza when it began to shake, according to the official newspaper of the Shenzhen Municipal Community Party Committee. Everyone inside was evacuated within 90 minutes, said local authorities. 

剛剛中午時分，位於深圳華強北，樓高73層的賽格電子大廈突然搖晃，原因不明也沒地震！樓宇內人群和樓下人群紛紛逃命！ pic.twitter.com/aoixkH6OeY
— 風再起時【香港挺郭后援會3】 (@dZnJUCdo4FlZqgd) May 18, 2021

Authorities investigating the cause of the shake said in a statement that no abnormalities in the building or the surrounding area were found. There were also no earthquakes in the area. 

As noted by The Guardian, Chinese media have reported, through a preliminary investigation, that the wobbling was vertical rather than horizontal and that it was likely caused by a combination of winds, underground rail lines beneath the building, and the stretch of steel caused by rising temperatures. 

The 21-year-old building is named after semiconductor and electronics manufacturer Shenzhen Electronics Group. It is located in the shopping district of Huaqiangbei, which is home to the largest electronics market in the world. SEG Plaza is the 18th tallest building in Shenzhen, according to the Council on Tall Buildings and Urban Habitat skyscraper database. 

SEG Plaza has been closed off to the public, and authorities have yet to say when or if the building would reopen.