Let's face it; The Simpsons isn't nearly as popular now as it was in its prime. Though it has lasted an impressive 23 seasons, the show's ratings have fallen around 20% since 2006, and Fox has decided that new episodes are too expensive to produce, so continuing to work on the series isn't as profitable as they'd prefer. So, it's been decided that if the show is to continue, the primary voice actors behind it (Dan Castellaneta, Julie Kavner, Nancy Cartwright, Yeardley Smith, Harry Shearer, and Hank Azaria) are going to have to accept a pay cut of 45%, reducing their income per year to $100,000. That may sound like a lot, but consider that before this cut, they'd be making almost twice that for the same amount of work.
Needless to say, the actors are not fond of this plan. They believe that they have contributed to the Simpsons' success just as much as higher-ups Matt Groening and James L. Brooks. They find it unfair that even if the show ends, the creators and executives will reap large amounts of money from syndication deals and the like, while all they'll get are residuals.
UPDATE: It has since been announced that regardless of whether or not a compromise will be met, the show's 25th season will be its last anyway.