Just out of curiosity I tried overriding a abstract method in base class, and method the implementation abstract. As below:
public abstract class FirstAbstra
If you did not declare SomeMethod as abstract override in SecondAbstract, the compiler would expect that class to contain an implementation of the method. With abstract override it is clear that the implementation should be in a class derived from SecondAbstract and not in SecondAbstract itself.
Hope this helps...