Would love to know myself as mine have been doing just the same. Have been growing runner beans for many years and have not experienced this before. I could only guess that it is the very hot weather as I have been watering regularly as always.

Same here. Makes cutting them up interesting doesn't it?

I have never seen an absolutely straight runner bean it is their nature to curl… I seem to remember the variety my parents grew was straighter but that was 55+ years ago! Any we have grown were curved… bit like a banana!