I said earlier "I have no idea where these melodies come from."
But I think maybe I do. Our mind is always picking up things that we hear, whether or not we are conscious of it. This could be music that we've listened to, or a bird's call, or something else we heard in nature. And then our unconscious mind processes it, and out it comes sometimes. Not very often in my case, but every once and a while. That's why listening is very important.

Then I always have to check whether it's just some other known tune that I've recalled. Or is it really original? It probably is never really completely original. One of the melodies that I came up with for someone else's lyrics, I particularly like (and other have also). After much thought I think it came from a small fragment of another song, but that other song was in a completely different rhythm, but it was about a similar subject. And then the rest of my melody was quite different. So it must have been just a small fragment of something I had heard before that set this off.

I think the unconscious mind processing the many experiences we've had has something to do with it.

Cheers, S. in Seattle