Your pitchers combined can only throw so many innings in one season (lets say 1300). As soon as you go past the 1300 inning limit none of your pitching stats will count. Say you burn through it at the beginning of September, your pitching numbers are frozen for the rest of the fantasy season. If a pitcher on your team threw a no-hitter the week after you've reached your limit, it wont help you any.
Why does this exist? It's to discourage teams from just dropping/picking up starting pitchers every day to try and rack up Win/K numbers and blow everyone else out in those categories. It forces the managers to actually manage their rotations and to use their pitchers when the match-up is most favorable. Just look in the bottom left-hand corner on your team and it'll tell you have many innings you've used, how many are left, and even if you're currently under/over innings at the pace you're going.