The bowls choose whomever it is they want in the order of the prestige of the bowl.
Did you forget just 10 months ago when several bowls passed over Utah and all the ute fans bitter complaining that they had to play in the Vegas bowl because of it? Based on where Utah finished in the pac12 if bowl placement had been based on that they would have been in a "higher" bowl, but those higher bowls didn't want them so they chose teams that finished lower in the standings.