When you eat a very low carb diet you aim to remove carbohydrates as your bodies main source of fuel. (myketokitchen.com)
Carbohydrates are the main source of fuel the body uses for energy when working out, and if you do not have enough carbohydrate stores from food that you have eaten, the body will begin to burn fat. (businessinsider.com)
This happens because glucose (from carbohydrates) is the easiest to convert into energy and hence is the body's main source of fuel, especially during exercise. (womenshealthandfitness.com.au)