One scenario I can imagine is that they have a route back to harbour plotted on the autopilot that doesn't go across the shipping channel, so rather than have someone on the bridge watching nothing they've got in the habit of having everyone sorting out the catch and gear, thinking that keeping an ear open for alarms from the electronics will deal with any unexpected traffic. That night they were particularly busy and no-oe heard the alarms. It had never caused a problem before and with the extra hand on the gear they get off work half an hour earlier each night...