Basically, every party member has a defense rating which factors in to their attempt to hold the line while you fight the Human Reaper.
If a particular character is loyal, give them a bonus +1 to their defense score.
If the average of everyone's defensive score is less than 2.0, people will start dying.
It'll start with anyone who isn't loyal, and after that it goes by a list. I know Mordin is first and Tali/Kasumi/Jack are 2nd, 3rd, and 4th (in some order, not sure which).
Generally, the best strategy is to send one of Mordin/Tali/Kasumi/Jack back with the crew, and then take two of them in your squad when you go to fight the reaper. This avoids them being involved in the hold the line segment, which is important since they're the biggest drain on the average.