(notation found here [Bach] and here [Schumann])
I have no real musical training, so I might be wrong about this, but it seems that for his fugue Schumann - like Bach - used a phrase that alternates between a single note and progressively lower notes. Bach's phrase (starting in the third measure here) alternates between A and various other notes (G, F, E, D, C#, and so on) while Schumann's (starting in the second measure) alternates between G and other notes (F, Eb, D, C, and so on). Similar figures occur later in both works, but I've used examples that are easier to see.
I knew that Schumann used the B-A-C-H motif in his 6 Fugues on BACH (it's the first four notes in the second fugue), but I hadn't realized that he also referenced other Bach work. I'd thought that the BACH in the title was meant to indicate that signature phrase (Bb, A, C, B), but I suppose it also indicates a larger influence. I'm not that familiar with most of Bach's organ works (I have only a two-disc set, and even that I haven't listened to that much), so there might be others quoted or referenced in Schumann's 6 Fugues that I'm oblivious to. BWV 565, however, is widely recognized (in contemporary times at least), so that reference was easy to pull out.
