In the last five years in the Big 12, only 13 units managed to field a defense that ranked in the top 40 of S&P+. Oklahoma and TCU managed to produce three such teams, Texas and West Virginia have each done it twice, and then Baylor, Oklahoma State, and Iowa State have each pulled it off once. For their defensive reputation, the Horned Frogs had a fairly long dry spell in 2015 and 2016 before they were able to crack the top 40 again last year (no. 16), and OU’s defensive reputation is up in smoke after finishing at 55th in 2016 and then going over a cliff in 2017.
The two years that Texas managed to finish in the top 40 were 2014 (year one with Charlie Strong) and then 2017 (year one with Tom Herman/Todd Orlando). If you look at those specific teams, or the 13 other top 40 defenses of the last five years, it becomes clear what it was about those squads that allowed them to play good defense. Namely, experience and particularly experience in the secondary.