There are two major causes of human trafficking. First, there is demand from people who wish to be brought from one place to another. Second, there is demand from people who wish to exploit others.
The first of these usually takes the form of people from poor countries who want to find a better life in a richer country. They often pay people to smuggle them into the richer country. Some of these smugglers use the people they smuggle as forced labor in the richer country. The fact that these people want to get to the richer country is a cause of human trafficking.
The second of these is seen most often in the case of people who are trafficked for sexual purposes. In these cases, the cause of human trafficking is the demand for prostitutes. The traffickers find people that they can use to fill this demand and move them to the places where the demand exists.
These types of demand are the major causes of human trafficking.