How population growth led to an increased need for food?
Consider a situation: you live alone on your farm and have a garden that gives you 200 kg of fruits and 250 kg of vegetables every May. That's enough for you alone, isn't it? But imagine you have a spouse. These 450 kg of food may still be enough. Now imagine you eventually have 4 kids and your and your spouse's parents moved to the farm, so there are 2+4+4=10 people. And they all need food. Initially, you had 450 kg of food for you alone. Now you have 45 kg of food for every member of your family. And someone may require more, taking this food from others. But the field cannot give you more than 450 in May, so you need to find new ways how to support the living of your family.
Something similar happens on a larger scale on our planet. This is how population growth led to an increased need for food.