Don't laugh ... I've just been out with a flash light to check and see if anything changed with the cooler temps. Nothing happening. Standing tall and still like all the other non-dancing companions.
We don't have poisonous snakes in our neck of the woods so my first thought when I saw movement from the corner of my eye was that I had disturbed a large insect like a dragonfly, and I assumed as it flew off it caused the movement of the plant. But then the movement continued and it was irregularly repetitive ... always in the same direction. So that's when I hurried in to get a camera with video capability.
Brought hubby out to see it when he woke a few hours later, but by then it was behaving like all the other plants in the bed ... moving slightly with the breeze that had picked up. It had put its dancing shoes away. He scrutinized the videos but his scientist brain could come up with no explanation either.
Very strange but we'll check it again in the morning and report back