You'll truly begin to understand the rivalry when you understand it's only 20% football, and 80% everything else in the state. They are the proverbial "spoiled sister" who has everything handed to them for decades while you work for every single thing you've ever had.
When their system was developed (almost 20 years after we were established), the PUF/ permanent university fund (state oil revenues) was established, directing 75% of those funds to build their system. The A&M college received roughly 20%, while the remaining public institutions received a pittance of the remainder. Colleges and new campuses open, grants, endowments. Enough of an advantage for you?
Even then, most of us can deal with a spoiled sister… until they start bragging about it.
Now, imagine having an athletic conference centered on your state (SWC), wherein outliers from surrounding states eventually depart. Any school proving to be better at "the game" gets summarily buried (SMU early 80s; Jackie Sherrill after winning too many years in a row).
Then you decide to cobble together a larger conference (Big12) with a dying entity (Big 8), seating yourself squarely in the Chair position. That pesky female governor demands you include her school (Baylor) or she'll cut off that oil money of yours.
You offer to negotiate with the networks on behalf of the entire conference; yet when you return, you've landed your OWN network instead. Everyone else is held hostage; and "no, A&M, you can't build a new stadium that will make it the largest in the state." When the architects make it happen anyways, you promptly announce within a week the proposed expansion of the stadium in Austin to resume its "rightful spot".
Aggies have enough and depart in 2012, setting off the largest period of growth for our institution in a century. Sister pans the move as foolish, keeps other members hostage as the conference slips further back from relevance.
Fast forward to today, and imagine the other members being forced at gunpoint to attend Sister's Christmas party at her home. The TV is on, and somebody suddenly notices that they see Sister on the TV at an even BIGGER party down the street! That's what the Big12 just had to put up with; Sister left her own party that she designed from the ground up.
It was an ugly, codependent relationship for decades. I would've been happy to have only met them on the field in occasional bowl games. Our SEC brethren just invited the good looking, psychotic sister back to the table.
Now they all get to deal with her.