Chinese accused US military of bringing the virus to Wuhan. Trump took that to be an insult to America and retaliated by calling it Chinese flu/virus.
(Thought I read that China then withdrew ambassadors from US but can't find a link so might be wrong)
Not certain of timeline but I believe the Chinese were retaliating to a high ranking US official who repeatedly stated during broadcasts that the virus was developed in a Chinese lab. Will need to check the timeline on that but I seem to recall the US starting this tit for tat with those unfounded allegations.