What were the reasons some nomadic people decided to settle down?

They discovered farming. When a society begins to farm they don't need hunt or gather because food is available from farming. With the introduction of crops people could live together in groups for protection and create a culture with government, art, religion, music, trade, and invent new things. Hunting and gathering took a lot of time and effort to get enough food to survive, but with that gone time could be spent on other things and improve the quality of life for people.