Why would you fire people on such a short notice? No need to hand over things? No interest in providing at least some time to look for a new job? EDIT: To clarify that a bit, I am interested in what the upsides are except for possibly saving some money. An orderly shutdown usually seems preferable to me over quickly killing the process, for both sides.
From the https://www.crunchbase.com/organization/stack-overflow, the first round was from 2010 - 7 years ago. USV will like to get their exit soon.