Not sure he "mumbled" about it. In his autobiography, "It's Only a Game," he lays it out there pretty clearly:
"Just about everybody I knew ---as well as a lot of people I didn't know---wanted me to go to LSU. I didn't know how to tell them I didn't feel comfortable in Baton Rouge, so I purposely flunked the entrance exam. I never much liked people who make excuses for their failures. I am not claiming I could have passed that exam easily if I had wanted to go to LSU. I'll never know if I could have passed it; I know I didn't study for it, I didn't care about it, and I definitely didn't want to go to LSU. And I definitely didn't pass the test."
He talks about his lack of self confidence and for an 18 year old kid, that pressure of "why don't you go to LSU" had to be pretty tough to deal with. Instead of just saying, "I don't want to. I don't like LSU" he threw the entrance exam to avoid the question. Knowing TB's presence today, and all that he achieved in the NFL, it's hard to believe these are the same person...but hey, we each come into our own in time, I guess.