Besides the common predators like big sharks and occasionally killer whales, humans are the top killers of dolphin. If dolphins could figure it out, we would be considered their worst enemy.
The human activities that kill dolphins are fishing because dolphins get trapped in the nets while they try to catch tuna or some other species.
Also, human pollution is a huge threat for dolphins causing them to choke by eating debris or by damaging their senses and making them easy prey for sharks.
The excessive noise product of human activities in the ocean is also a threat to dolphins, as noise can kill dolphins or at least confuse them causing massive beach stranding.