donors. each year those seats are thrown into the pool and if the top donor wants some, he gets some... it goes down the line until they're all gone.
People have been donating and paying for season tickets for years. They've been made promises about their seats. It only makes sense that BYU will honor these commitments.
Still... I'm pretty sure the upper parts of the lower bowl are available each season (this may change next year though) - not all of the lower bowl seats are called for.
Note that this coming season's tickets have already been allocated... So you're out of luck this year.
You might want to speak to someone at the Cougar Club. I think they'd give you a decent idea of the donation amount needed to be at the right spot in line to get some lower bowl seats next year.