Jyn Erso (Felicity Jones) and the Rebels came into contact with Saw, Bokan and their pals after they crash-landed on the moon due to an electrical storm.
But this time around, Bokan was part of Saw Gerrera’s small crew that used a moon as their hideout.
Saw had actually modified his ships so that they could handle such an environment, but every other ship that tried to infiltrate it crash-landed.
This is why the Empire had never been able to find Saw Gerrera.