My butternut did the same thing to my surprise as well last year. I managed to gently guide one back into its bed. One escaped out the garden fence and was, within the week, lunch for some critter.

And, yes, they will grow up a trellis since the other butternut grew across the path and climbed its way up the tomato trellis.

It always amazes me how squash manage to almost overnight go from puny little things to garden devourers.

Thanks, ali-b. Since I wrote I found a 6-ft. length of 3-ft. high open steel fencing, the leftover from fencing the potager itself, and staked it in front (south) of the escaping butternuts. The long vines look a bit offended by being curtailed, but I'm hoping that with some gentle guidance, they'll continue to grow, but to the north. Or grow up the fencing.

Carol