Answer: The main cause was the fact that there was a mix of a lot of races, skin colours, birth places, etc... This sparked the need to distinguish between each person and classify them, which affected the way they were treated, their rights, taxation and other aspects of their daily life.
That is why, a Spanish person that mixed with a Native American would be considered "Mestizos", while a black person mixed with Native Americans would be considered a "Zambo", meaning that they would have almost no rights and treated as slaves.
