I have a similar problem with the starting adventure for new characters.
My players have an Imperial Navy Captain and a Scout (ex Trader). I have the 5 Marches adventures and wanted to run Mithril (they'd done hign n dry as an intro with other characters) but I keep wondering why the characters are going there or even why the 42 year old Captain of a Navy ship would decide to quit the Navy in the first place.
In general, why do any characters decide to leave their old career and just travel around looking for adventure?
Just looking at real life reasons why people leave the Navy at that point:
1) promotion isn't gonna happen. Its not reflected in the chargen rules, but once you get around Commander/Captain level, the jobs above you start getting pretty thin on the ground. And, many of them ARE on the ground if you get one at all. Like desk jobs, not commanding ships.
2) Got your pension. Time to go make some REAL money/enjoy life/control your own destiny.
3) Got a better offer from something in the civilian world.
4) Demobilization because war's over/budget cuts/radical peace party takes over local government
5) Bad boss/sick of it/all the reasons people leave jobs at any other time.
As someone else pointed out, this is really the player's problem to figure out. People always ask "why is my character adventuring if they have X money/are a retired admiral/whatever." And that probably makes sense to them as a question, since we are people who play games about going on adventures, not people who actually go on adventures (generally speaking). But your character isn't like that. Your character has a reason they are out on the sharp edge, risking their life trading on backwater planets or doing crimes or just going places sane people don't go.
Classic Traveller, you had to roll for re-enlistment in addition to rolling for survival. So, usually, you got out because that was the only option. Though sometimes you rolled a 12 and you stayed in whether you wanted to or not. Mongoose Traveller combined survival and re-enlistment in the mishap table.