If food is not equally distributed, some groups will receive more than others. For example, food is very poorly distributed across the world. Countries like the U.S. have a surplus of food while poor nations in Sub-Saharan Africa often don't have access to enough. A group may have so little access to food that they fall into a famine.
Drought and flooding can destroy agriculture. A drought is a lack of rain that will not allow crops to grow and will cause animals to die. A flood can also be disastrous by submerging fields, drowning livestock, and destroying infrastructure. When livestock and crops die, a famine will occur.
An increase in population just means more mouths to feed! The more people there are, the more food needs to be produced. If food production cannot keep up, a famine can occur.