While it might be tempting to think it was a Texas led conspiracy against the Huskers, and the argument is the rest of the Big-8 went along out of jealousy; that would be an overly paranoid reading. BTW, this sort of "big school" enforcement /boycott threat is what allowed Jim Crow ("gentlemen's agreement") policies occur even outside of places where state laws required segregation.
Texas always had issues where they couldn't get some players in while less "academically stringent" schools could. So they basically dictated that the SW Conference would not take these players (Partial Qualifiers were only a later manifestation of NCAA rules that dated back to the 1960's*). So UT wouldn't want to play against players they couldn't take. So they didn't want Houston, or Tech, or aTm to be able to take players academically that UT couldn't. They even would make this argument to their fans as to why they would NOT consider joining the SEC (which the fans would have preferred).
*In the late 1960's the NCAA put in a minimum requirement that for a school to be able to take a player, they would need to predict a minimum Freshman GPA of 1.6 on an admissions test (ACT or SAT). The Big Ten put their limit at 1.7. Further, these predictions are going to be school based by definition. So an Iowa or Nebraska would likely have a better chance of qualifying a kid than Michigan, Ohio State or Northwestern; it would be easier to get in a kid with a low test score. Large state "flag ship" Universities tend to be way more competitive for admissions than small state's schools; Ag colleges easier to get into than the "doctor and lawyer" schools.
That 1.6 vs. 1.7 was a huge issue in the late 60's early 70's. Had a tOSU fan claim we got MNC in 70-71 because of that.