Many big cats are unfortunately endangered because of our (human) influence on their habitats and natural way of life.
Here are a few reasons why more and more big cats are being endangered:
Poaching and Hunting
Poaching is when humans kill animals to trade their different body parts. Lots of cats are poached for their skin. Of course, this is illegal but it is often hard to catch poachers since they go deep into the jungle, animal reserves and cat natural habitats to kill the animals. Other times the cats are hunted to keep them from preying on livestock.
Big cat populations are dwindling because they have less food to eat (prey) and thus less of them survive since their is increased competition for food.
Habitat loss and environment change
Because of human driven deforestation, building and agricultural expansion big cats are losing much of their natural habitat. Global warming also affects the natural habitat of big cats and makes them much more vulnerable to their environment.
There are other threats to Big Cats that are more area- and species- specific but these are the main ones affecting big cats on a whole.
Sources: Wildlife Conservation Society wcs.org and Seethewild.org