A conspiracy theory is a symptom of what people already suspect and believe about those in positions of authority. In this case the suspicion is that the epidemic is a result of a preventable accident and the incompetence and corner-cutting of the lab is being covered up to save face. People believe it because safety endangering corruption, saving face, and covering up industrial accidents are all very prevalent in Chinese society. In recent memory there's been high profile cases like the tainted milk powder, the HSR accident (that was literally buried), the explosion in tian jin etc.