Personally, I don't understand why a musician would take one bit of a tune and marry it with another, particularly with the two tunes mentioned as they are both crackers in their original form.
Well, one possible reason, probably responsible for a lot of evolutionary change in folk music, is simply an inability to remember a tune entirely or exactly as one heard it. Why deny yourself a good A section just because you can't remember the B section you heard it with? Why not graft on a new B section or one from another tune?
I think you are quite close to the truth here, JB.
Walter Bulwer was quite advanced in age when he was recorded playing the two tunes which make up Shipdham Hornpipe. It's quite possible he muddled them up, which he might not have done when he was a younger man.
I think there may be something else going on there as well. Correct me if I'm wrong, but from what I know of the recordings, these are the step dance tunes.
I also understand that these players often played for dance competitions where they would be playing for many different step dancers. I've experienced something similar, albeit in a more attenuated form, whilst playing for rapper tours, and especially playing for rapper workshop tours. Playing for team number three at the fourth dance spot on a long weekend of workshops can be an "interesting" experience.
When they get to know you, dancers are not backward in coming forward about the tunes you are playing. What works, what they like to step to, bits of tunes they dislike for whatever reason. Even tunes that you're not supposed to play in that context.
When what you're after is the correct rhythmic feel and phrasing, where the 32bar or 48bar structure is irrelevant and the actual length of the tune sets is not an issue (or, rather, it's an unpredictably variable one), this mix and match approach is quite possible and even likely.
I've experienced this over sort of thing short (but intense) periods with sets of people that I did not otherwise know. I can only imagine how those "pressures" would affect a players over years, with people who they would often know well.
What starts as an innovation becomes a habit and then simply becomes the way it's done without much comment amongst a community of players and dancers.
There are also many tunes where this mix and match does not happen. So, I suspect that as with much of folk music this is down to the
