WU's released at any other time other than the second Tuesday of any month, are known as out-of-band update releases. Microsoft does do this on occasion, usually after multiple consumer and enterprise complaints about a specific windows security updates causing issues with user's systems.
So, if one were to come in today, it would be out-of-band and would normally be a fix to a prior bad WU. Otherwise, an out-of-band will occur when a newly discovered security vulnerability it addresses cannot wait until the normal Microsoft date.
Rare, but does happen from time to time.
