Matthew Yglesias wrote a whole thing on how the idea initially became politicized in the US. The short of it was that when Trump was downplaying the danger of human-to-human transmission early on because his friend Xi told him that wasn't happening Senator Tom Cotton started talking up the idea that the Chinese government might be lying about because it escaped from a government lab. But then there was a game of telephone and it turned into the idea it was a Chinese bioweapon.

https://www.slowboring.com/p/the-medias-lab-leak-fiasco