The atmospheric drag on a planet could make its orbit wobble enough to cause a cascade effect through the entire system. As soon as one planet wanders a bit to

The atmospheric drag on a planet could make its orbit wobble enough to cause a cascade effect through the entire system. As soon as one planet wanders a bit too far out of line it'll smack into others going the opposite direction and the debris will take out all the others inside of a few revolutions. Interesting, but highly risky.