One of my followers posted that the plants are sunflowers and I think she's right.
Here's a post with a picture of the sunflowers I grew in 2010 and the plants look exactly alike. I don't know why I didn't recognize them...except that I had it in my head it was a squash plant, until it didn't look like a squash plant anymore.
I have no idea how those flowers got planted in that spot. The 2010 flowers were at the opposite end of the garden, so I don't think there's any way those are seeded from the ones I planted.
Oh! It just occurred to me! I've been feeding the goats black oil sunflower seeds and that is where I had Tom dump the load of bedding and goat manure that we shoveled out of the stalls.
Thanks to Maree Clarkson ( http://www.blogger.com/profile/16274329795088706720 )for identifying the plant.
Now I have no butternut squash. What a pity, but I will enjoy the accidental beauty in the garden.