There are two competing theories.  The first theory has been circulating for months in China. A US Military Research Center put a pathogen into e-cigarettes which were then transported to Wuhan’s 2019 Military World Games. 

Society Garlic has uncovered a second theory which hypothesizes that the virus originated from famed US slaughterhouses known for their ethical and hygienic treatment of animals and their remains.  The virus may have jumped from a pig to a cow and the rest is McHistory.