Yeah, I noticed that the rival tends to be someone you had close games with last season unless you promoted or relegated.
My RNG comment was referring to the TV game selection; you won't always play 2 TV games against the same team in a season, but sometimes you will, and there's a chance that team will be your rival as well. Don't blame the RNG for doing it's job and randomly deciding that your rivalry games should also be TV games.
I think it's more than just the last season, though. It has to be, because I'd have a different rival otherwise.
Two seasons ago, I split with a new promotee (Libertines), winning the home game by 9 and losing the road game by 8. Meanwhile, the team that ended up being my rival (DC Dribble Penetrators) I lost to by 29 on the road and 11 at home, but we had split the home and away games the two previous seasons.
And last season, my games with Libertines were both won by the home game, with almost identical scores: 84-74 and 85-74. Through two seasons, we've split games and scored the exact same number of points. Meanwhile, I swept DC last season, by 13 on the road and by 21 at home, and the rivalry hasn't changed.