its just luck. Killing them with a super or under the effect of BWI will increase the drop chances. Another neat trck it to do the Shrine of Oyrx mission. Go through Kill Sardok who drops either a BWI or Silken Codex (sometimes more than one), Don't blow up the machine, just go back into orbit, then reload the checkpoint, Kill Sardok again, gets drops, then rinse and repeat until you get the desired amount.