If the pods alone went brown, you probably simply left it too long before picking. Scarlet runner beans should be picked small, and there are some weather or fertilizer issues that can stunt the pods.

If the whole vine went brown, something drastic happened! My first suspect would be root damage from moles, voles, or grubs. The same thing can happen with a too vigorous weeding. If it was in late summer or early fall, I would also suspect an early frost: you might want to plant earlier this year. One year, my neighbor's asparagus bean went brown suddenly, and it turned out that he had cut them off at ground level with a string trimmer, while edging his lawn! Sometimes it's a case of "We have met the enemy, and he is us." (Walt Kelly). I hope things work better this year!