Because the Iron Lords were losing the fight, and the Siva nanites had infected their armor. She realized that they couldn't win, but that they could succeed in trapping the Siva inside of the bunker. So she waited to see if Saladin would be successful in destroying the Siva that had escaped to the outside. When he did succeed, and turned to join the rest of the team inside the bunker....that's when she waved him off. Shut the bunker door, and set off the bomb. There's a kind of traumatic bonding that happens between people who fight side-by-side in war. People who have experienced say that you don't fight and die for ideas or beliefs. You fight to protect the life of the guy next to you....and he fights to protect yours. A bonding that can motivate some people to astonishing acts of bravery or self-sacrifice. From charging machine gun emplacements under fire....to throwing themselves on a live grenade to save the rest of their unit at the expense of their own life. I think there was an element of this....but I think there was also a deep personal (and I believe intimate) relationship between Jolder and Saladin. I think she sacrificed herself (and the rest of the Iron Lords) as much to save HIM....as she did to defeat Siva. The fact that he knew exactly what she was planning we she shook her head...and he panicked and started racing even faster to join her is telling. The look she gave him as the door was closing was not one of fear....but one of sadness, regret and trying to burn his face into her memory before end. The kind of look you give to someone you love, that you know you will never see them again. Then there was the devastation he clearly felt after it happened. Or own Guardian seems to understand the nature of their relationship, because after the battle in the Siva chamber, of all the Iron Lords we bring back HER helmet to the Iron Temple....and its the returning of something of her that seems to bring him some measure of peace. But no, without understanding the bonds that people form to one another....especially bonds in time of war and great danger....there is no real way to understand why she did what she did. Why she sacrificed herself for the sake of others.