Here's why:
* "Keen" in this context means eager or ready.
* "Fields" refers to the land where crops are grown.
* "Before monsoon" indicates the time period.
The phrase "keen for harvesting" means the peasants were eager and ready to harvest the crops before the monsoon rains arrived.
Here's how the sentence would look with the correct phrase:
The peasants were keen for harvesting the fields before the monsoon.