 What caused the ship to leave its planet initially?
When nobody aboard ship remembers where it's going, how can they tell when it has arrived? But something soft had cushioned the impact—something which had come into being just for the moment and then abruptly passed into non-being again, something which was as impalpable as air. The people had decided that they did not know where they were going, or why, and that it was unfair that the elders alone had this authority. A section of it had expanded outward and assumed the rectangular shape of the viewport, and its size as well.
 The reason is never disclosed.
 There was a shortage of women, and the main characters were sent to find mates.  The reason is never disclosed.  The planet they were from ran out of viable resources.  They are explorers who got lost, and their fate was to drift the universe.