The service has been there for quite a few versions - and it's just related to internal re-organization of Avast code (it was decided that some functionality, previously inside AvastSvc.exe, would be better placed in a separate service, for various reasons).
However, I would really expect that a program update has happened on your machine, it cannot magically appear without an update. Well, the only other explanation I can think of is that your installation was somehow corrupted before (this service was missing), and the installer found out and triggered a "repair" - which created the new service.