A chicken's back end is supposed to slope upwards at an angle from between the legs to the tail, so that when they poop it drops clear of the feathers below. If the bird is carrying too much fat or has swelling around the vent there is no longer enough overhang for the poop to drop clear and it starts to soil the feathers below. Once some poop sticks, a build up rapidly follows. Swelling around the vent can be due to fat deposits due to too much carbohydrate (not enough protein) or certain laying disorders like internal laying or salpingitis. My guess would be that your Red Ranger girls are probably carrying too much fat due to a dietary imbalance. Heavier birds are more prone to it, especially if they do not get enough exercise.
What do you feed them including main feed, scratch, corn, treats etc?
