What is one contribution that led to the Abbasid Caliphate's decline?

The early Abbasids were able to use the energy and capabilities of their entire population, including both Arabs and non-Arabs, both Muslims and non-Muslims, to build a centralized state with an effective army and bureaucracy. Eventually, however, the Abbasid bureaucracy fragmented into self-serving factions, and, at the same time, the army regiments started to fight amongst themselves in competition for privileges and handouts; the troops consisted largely of Turkic slaves. This led to a loss of popularity for the government. Since the Abbasid dynasty was Sunni, Shiism became the main ideology of popular resistance.
Both the army and the bureaucracy became utterly corrupt by the second half of the ninth century CE. In the 860s, the ongoing struggle between various government factions facilitated a large uprising of East African slaves (the Zanj rebellion 869-883) in Southern Iraq. It took the Abbasids many years to defeat the rebels, and during the course of this conflict much of Southern Iraq was devastated, including Basra, the main Abbasid port in the Persian Gulf. Meanwhile, the Abbasid empire started to disintegrate as governors of distant provinces became more and more independent. In the tenth century, the Abbasid Empire declined economically; this led to a shift in trade routes from the Persian Gulf to the Red Sea, East Africa, and the rival (Shia) Fatimid caliphate in Egypt.
In 945, the Persian Shia dynasty of Buyids conquered Baghdad. The Abbasid caliphs became the pawns of the Buyids and, after 1055, their new conquerors, the Sunni Seljuk Turks from Central Asia.