How does an animal become a endangered species?

A species is usually considered threatened as it is becoming endangered, that is if it doesn't meet the requirments for endangered from the time of it's first study, or they might be considered endangered before a Listing is enacted or developed. As with the Humpback Whale, they were already considered endangered before the Endangered Species Act of 1973 was enacted.Some of the lists have different listings and levels for species. IUCN and CITES are both different from the Endangered Species List.
Most endangered species are endangered because we are killing them for there teeth, skin and bones or there habitat is being destroyed or there isn't enough food for them.