Why declare an interface as abstract?

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 This modifier is obsolete and should not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.