Why people keep shouting "China number 1" at the start of a round in PUBG

Going a little offtopic here - The history of popular battle-royale game mods date back to 2000

when the Japanese film Battle Royale was released. This genre of gaming did not gain a lot of popularity until Hunger Games was released in 2012.

Battle Royale was adapted from the 1999 novel
of the same name by Koushun Takami.

The origins of PUBG

Shortly after the release of the film The Hunger Games, a Minecraft mod for battle royale called the "Survival Games" was made. Popular youtubers AntVenom and SkyDoesMinecraft tried to make this mod popular.

Other popular games of the same genre includes mods for the game DayZ and then ARMA 2. Brendan Greene was then working as a consultant for H1Z1 when he decided to move in as a creative director at Bluehole.

Chinese Civil War

This war was fought between Republic of China and the Communist Party of China. which led to an ever lasting deadlock between the two sides of the Taiwan Strait, Republic of Chinain Taiwan and the Communist Party of China in mainland China both claiming to be the rightful government of China.

This tells us that the people in Taiwan and mainland China don't love each other too much.

Taiwan number 1

H1Z1 streamer AngryPug while playing against a team of Chinese player Em0 started shouting "Taiwan number 1" right after being killed by them which clearly pisses the Chinese team off. This video became quite popular among the Taiwanese and the Chinese needed to reply to it in some form. Take a look yourself. Go to 2:10 when AngryPug dies.

Another instance of a similar incident where AngryPug provokes his enemy

China number 1

The PlayerUnknowns Battlegrounds game was released in the early 2017. The genre was already made popular by H1Z1. Most of the people at PUBG had already played H1Z1 in the past and they knew what responses one can generate with "Taiwan number 1" or "China number 1". And since the default Chinese reply to "Taiwan number 1" is "China number 1". That's from where the phrase came in existence. Sometimes people try replacing the country with their names but this is where it all started from.