According to the Wiki:
If a villager has enough food in one inventory stack (6 bread or 24 carrots, potatoes, beetroots, or 18 wheat for Farmers only) and sees a villager without enough food in one inventory stack (3 bread or 12 carrots, potatoes or beetroot for non-Farmers; 15 bread, 60 carrots, potatoes, or beetroot, or 45 wheat for Farmers), the villager may decide to share food with that villager.
My guess is that you reached the limit for when villagers will share food with one another (the villagers had 12 or more carrots or if they are/were farmers 60 or more). This is why they stopped.
It is also possible that the population exceeded the amount of valid doors in the village:
Villagers will mate depending on the number of valid doors. If "willing" (see below), villagers will mate as long as the population is less than 35% of valid doors, rounded down. The type of villager that spawns is not dependent on the villager's parents.
A valid door is any door within the village radius where the number of "outside" spaces within 5 blocks in a straight line on one side of the door is not the same as the number of "outside" spaces within 5 blocks on the other side of the door. A space is considered to be "outside" if it has nothing but transparent blocks above it all the way to the sky.
The villager's must also be "willing" to mate:
Villagers may become willing when the player trades with them. Willingness is granted the first time a new offer is traded, or at a one-in-five chance on subsequent trades. Green particles will appear if the villager becomes willing by trading. This will not cause them to immediately seek out a mate, however.
Villagers can also become willing by having either 3 bread, 12 carrots or 12 potatoes in one stack in their inventory. Any villager with an excess of food (usually farmers) will throw food to other villagers, allowing them to pick it up and obtain enough food to become willing. You can also throw bread, carrots, beetroots, or potatoes at the villagers yourself to encourage breeding. Villagers will consume the required food upon becoming willing.
I would check all of the variables above and make sure you aren't exceeding any limits. There is no evidence that I could find stating villagers get bored of crops.