There are few theories behind the outbreak of the disease in China. But, I will tell you the one which in my opinion is the most genuine reason. There is an old wet market in Wuhan, China that caused this outbreak and according to the investigation, there were several animals that were kept together including bats. Thus, this virus is probably emerged from the bats and transmitted to other animal meats and in turn caught by the person who bought the meat. The first case was reported in late December 2019, but the medicinal official who suspected initially that it is different than normal flu was silenced by the Chinese government.
Just follow the instructions given by the government of your country for now and stay safe at home. Stay awesome!