The rise of Islam, Ottoman Turks siege and the spread of Catholic Christianity were causes of trouble to Byzantine Empire.
The Crusaders were the armed forces sent by the Western Roman Empire who were in favor of the spread Catholicism and their main aim was to capture Byzantine which was following orthodox Christianity.
Islam rulers also were intended to capture the Sacred land (Israel and a part of Palestine) and destroy Christianity in Constantinople. Many Islamic rulers were easily able to destroy Byzantine Empire due to its internal Civil wars and disintegrated military system made Byzantine to lose themselves to Ottoman Turks.