In the cfg's you can script a certain "effect" to take place once a certain part of a ship takes an ammount of damage given in %.The crew-men if I am right are blown off when you destroy a cannon/machinegun or the command tower of the ship.
Edit:Here's an example Teddy Bar gave me to make me understand
You can find these script settings in the zones.cfg in the data folder of your game.
There you can see the ship , sub and plane dmg zones and some "effects" like these:
Effect1=#Fire_small, 40
Effect2=#Small_splinter_explosion, 100
This for example means that when the ship part reaches 40% damage it will start to burn.When it reaches 100% damage,you will see an explosion.
If instead of Effect2=#Small_splinter_explosion, 100 you write Effect2=Small_crewmans_splinter_explosion01, 100 then you would get the crew-men flying off the ship when it is destroyed. Don't however change anything else unless you know what you are doing,because you might disable the NYGM dmg model.