Why declare an interface as abstract?

Interfaces and interface methods are implicitly abstract even if not declared as so. So there is no need to explicitly specify it.

Where did you come across the chunk of code you have posted, any old java code base ?
This is what the JLS has to say :

9.1.1.1 abstract Interfaces:
Every interface is implicitly abstract. This modifier is obsolete and should not be used in new programs.

9.4 Abstract Method Declarations:
For compatibility with older versions of the Java platform, it is permitted but discouraged, as a matter of style, to redundantly specify the abstract modifier for methods declared in interfaces.