There are many reasons why peregrines are endangered, most are due to human intrusion. Many peregrines were shot down in England, because they continually attacked and then ate carrier pigeons. Carrier pigeons were given secret messages to share with France and the allied forces during the war. Hundreds of peregrines were killed so messages could go through.
Peregrine falcons nest on cliffs overlooking water or open spaces. They hunt by swooping down on their prey over an area with few hiding spots. Unfortunately for the peregrine, people also like to build cities near water. Due to city development peregrine habitats were destroyed to make way for skyscrapers and other buildings. Humans were destroying peregrine's hunting grounds also. With the majority of their nesting and hunting area gone they started dying off.
Lastly, a pesticide called DDT killed many baby chicks before they had a chance to hatch. DDT would prevent the mother peregrine's body from producing the calcium carbonate needed to make the egg shells strong. When this happened the eyases would overheat and die. Often the egg shells would crack before the chick was developed enough to survive outside the egg.