In a totally random non-transparent manner, and no I am not joking or exagerating that is the way it is.
Some guys are lucky and get what they want, sometimes "suspiciously" fast, others wait years, run out of patience and leave.
All true I'm afraid.
However, when you are at a base, in practice you are left alone, unless the base were to shrink or close. Also, when you come up for command, if you are co-jo, you have no "grandfather rights" and could end up having to move unless your base is expanding/ someone else is moving/ you speak to the powers that be when they are having a good day.
All rather random, as many will testify, I personally was very lucky, but I know many for whom the coin fell on the other face too..