This is also a very good sign considering some price increases as well and a decrease in competition this season. I think what you are seeing is an increase in alumni base (and your true fan base) year by year.
It would be nice to see the increase in sales continue over the next 10 years. By the time BHNS is paid off, season ticket sales could be over 30K.
With teams like Miami, Georgia Tech, and even Navy on future schedules, I suspect we will see continued growth, if not even a little more rapid.