The way 5e describes this oath, I would expect any unprovoked attack by such a paladin to break their oath. Oath of the Ancients paladins are supposed to represent good and light, showing mercy and forgiveness ( etc, etc ). Defending against attacks is fine, but initiating combat is arguably wrong as this subclass.
If the goblins initiated combat, but you got the first blow due to initiative, that would be a bug if it triggered oathbreaker, in my opinion.
