Some people seem to think the seats are sold to 91,000 Joe The Farmers. The fact is, a huge number of seats are bought up by rich boosters who sell them or give them away to employees, friends, family, etc. When the boosters start to get bothered, large number of seats are in jeopardy.
Also, the notion that there are a million eager fans waiting in line for season tickets is utter nonsense. They struggled to sell out the east expansion seats until well into August, and I believe they accepted donation amounts below the initial suggested amounts. They even scattered in some visitor seats to the 600 section to fill gaps.
I think they will probably be OK for 2014 but it will take some serious marketing effort and they will accept lowered donation levels.