I’ve seen this phenomenon many times over the years, while walking the dog out behind our house. The cables wiggle between these two particular power poles, but NOT between the adjacent poles (or any others, along this pole line). There is no wind, no earthquakes, no herds of animals or large vehicles/machinery anywhere nearby when this is happening. I honestly have no idea why this happens sometimes. Thoughts? I mean, sure, it’s probably ghosts, right? But any other explanation would be appreciated…
Did you measure the distance between the poles? I suspect it’s different from all the other spans, so this one happens to have a resonant frequency that exactly matches whatever vibration source is already there (could be the tension too). As for sources of vibration, wind is probably it, even if it’s not strong. If it just happens to create the right frequency, the cable will vibrate just like a violin string.