From WhiterValkyrie in the Xbox guide:
Your crew will (possibly) mutiny when their morale drops to 1. Achieving this is very easy – just take away all the food and water from your ship’s holds and their morale will be dropping like crazy.
Some random events at sea will also allow you to drop some morale.
After your morale reaches 1, the next day, a random even will pop up.
You must then choose to beat up your crew (fight them) during the mutiny to get the achievement.
The achievement pop when you come back on the world map.