While the system is somewhat mystified and needs to be explained, another issue is that there is no politicking with fellow vassals, it would be quite fun to engage in some mild intrigue to undermine other ambitious vassals and ensure you might get a better slot in the running in the future, but nothing like this exists at present.
From reading other posts, it seems it has something to do with: 1) whether or not you have a fief, 2) proximity of the captured area to existing fiefs, specifically castles only apparently, 3) whether or not your lord is a greedy mf, 4) participation in the army (not necessarily the fight per se), 5) clan rank is also apparently heavily weighed as well.
The main problem I have with this system is the relations issue with lords and nobles at present, some of it being cheesy (1 denar to get charm up), and others being broken. You should be able to grease the wheels if you will with fellow vassals, including your liege, through 1) your actions, 2) coin, 3) bailing them out of difficult situations, giving significant relations boosts for any of these items. It should not be unusual for an active lord/lady to get great relations with everyone assuming 1) you aren't voting against them constantly and 2) you have been doing things for them, helping them out. Due to the semi-broken relations system, coupled with the arcane mechanics of being in the running, and adding the extra lack of intrigue, it makes the system incredibly random and obnoxious at present.