Spoiler: Loyalty missions Completing Jack and Miranda's loyalty missions triggers the fight. I made the mistake of doing them right away in my second play through and got burnt because my paragon wasn't high enough. I was never able to fix it because you apparently need 100% paragon to get Miranda (or Jack?) to forgive you. The other weird thing I noticed was my paragon play through had more chances to be evil than my paragade one. For instance, I was given the chance to betray Samara on Omega station as a paragon but as a paragade I automatically sided with her. Go figure.