The Mountain West title game is set between the Air Force Falcons and San Diego State Aztecs, but since the two did not face each other this year home field advantage will be decided by some of the old BCS computers which are somehow still a thing with the College Football Playoff.
The exact computers are the Anderson & Hester, Billingsley, Colley Matrix and Wolfe computer rankings, and the composite will be used to determine who hosts the Mountain West title game.
San Diego State is currently undefeated and Air Force has one loss in league play and since they do not play each other and neither are going to be ranked int he committee's top 25 the next step is to go to these computers.
The idea that the best team to host the title game to be protected makes sense, but only when a team is in contention for a New Year's game and ranked in the College Football Playoff rankings. With no head-to-head matchup and neither team ranked means the next tie-breaker goes to these computer rankings, and not the logical order of conference record, divisional record and then common opponents.
After the Week 12 games, Air Force who now has a commanding lead over San Diego State with an average ranking of 46.5 to 55.8 in the composite. All signs point to the Falcons hosting the Aztecs despite them being undefeated in the conference, assuming Air Force wins.
This likely will be discussed in the off season and it can be reasonably expected that the tie breaking order gets adjusted for next year.