Interesting stuff! I wonder if it's simply something to do with trying to establish a bit of rivalry between the (random) clubs that get paired up? Eg. Hibs v Queens isn't an attention-grabbing matchup if they play each other in September then March or something, but if the two matches take place quite close together, the press can talk about "settling scores" in the second of the matches. In a similar way to European ties, where maybe the tackles were flying in the first leg, or one or both teams feel aggrieved at the result, and the second leg gets portrayed as a grudge match by the media. This might also help to keep attendances high for midweek games.
Otherwise, I'm stumped!