Since there are way more that 2300 reserve seats on the home side of Indian Stadium, it makes sense that the ticket office would give existing season ticket holders the option to buy GA tickets to MCM once the 2300 MCM reserve seats sold out. Obviously, those 2300 tickets are coveted by the season ticket holders every time MCM is played in Nederland, as means that they don't have to arrive several hours before the game to stand in line to get a good seat.
From what I can recall, season ticket holders have always been given the option to buy GA tickets for away games that do not offer reserve seating on the vistor's side, which I believe is every away game with the exception of MCM. What is left over, or any additional allocation from the host school, goes on sale the week of the game.