Well we never really get the story from the aliens point of view so we don't know what really happen on the ship but from what I understood they run out of fuel. That liquid that transformed Wikus, made all the tech work once again. The one it took them 20 years to make one vial. They somehow lost it and then the ship was dead. Everything (except antigrav) stopped working and aliens almost starved to death. By the times humans got to them they were already complete mess. And then it turned out they got stuck in the middle of nowhere without any decent fuel station. No wonder they attitudes were so fatalistic.