The only thing I do know is as you flush out your gallbladder/liver, you release toxins. You liver is the seat of your emotions, so as the toxins are released, you sometimes go through a LOT of emotions.
Maybe the same thing is true in a sense with oil swishing. Could it be toxins are pulled from your liver? I honestly don't know.
Are you having the same nightmares or are they all different? Afterward, do you have a peace about you or a better sense of well-being?