It's purposely evolving diseases to spread faster or be more dangerous, for the sake of research. As I understand, it's at least a bit controversial. So maybe there's not as much of it going on as other research? If so, there probably wouldn't be as much opportunity for it to escape. But now that it has (per the hypothesis), it's ready to be very contagious right out of the gate. Thus, pandemic.
SARS-COV are classed Category C pathogen by CDC, in line with Hanta virus. SARS-COV is documented as a viable bioweapon, precisely for the things we have seen in the last year, and studied as such by all capable militaries around the world.
The hypothesis is that this lab (and labs in Iran, China and Iran share biowarfare research) was conducting gene-targeted coronavirus research. Using proxy DNA-testing companies serving Western populace to get their data. A good weaponized coronavirus would have an extremely high R. It would look similar to the flu in the first stages. Then at a later stage (after two weeks) it would deliver a "payload" in the brains of the targeted populace, stopping breathing or causing haemorrhage. The non-targeted races would just have a flu and contribute to the spread. Other engineered viruses focus on plausible deniability, straining the hospitals with patients with vague symptoms, hard enough to visit the hospital and contribute to the strain on public services, soft enough not to actually kill them. It would throw the targeted country into chaos and unprepared for a war.