Sparrows love using string fibres to line their nests, and if you tie up your beanpoles or sweet pea frame with briown string, they will fray and shred every fibre right back to the knot. The knot will then come undone (and they'll then happily take the rest of the string. If you've come back to the plot to find your carefully constructed bean fram all askew, this may well be the reason.
Sparrows don't recognise green string as string, so they leave it alone!