Source : Free On-Line Dictionary of Computing

hysterical reasons
     
        (Or "hysterical raisins") A variant on the stock phrase "for
        historical reasons", indicating specifically that something
        must be done in some stupid way for backward compatibility,
        and moreover that the feature it must be compatible with was
        the result of a bad design in the first place.  "All IBM PC
        video adaptors have to support MDA text mode for hysterical
        reasons."  Compare {bug-for-bug compatible}.
     
        [{Jargon File}]