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The office of amir of Mecca was the most prestigious Arab-Islamic position within the Ottoman Empire. Its holder was the guardian of the holy cities of Mecca and Medina. Although the Ottoman governor of the Hedjaz was in charge of administrative affairs and military security in the region, the amir of Mecca held an autonomous position as the individual responsible for maintaining the sanctity of the two holy cities and ensuring the proper and safe conduct of the annual pilgrimage. In addition, the amir of Mecca was selected from among those families claiming direct descent from the Prophet and so bears the honorific title of sharif. Abdülhamid II appointed Husayn ibn Ali, a sharif from the family of Hashim and scion of the 'Awn family, to the office of amir in 1908 (Probably, There's a bit of debate about this). He was an ambitious man who guarded his autonomy and strived to preserve the title of amir to his family. He distrusted the Young Turks that had been taking over, but is willing to work with them as long his autonomy was preserved and his authority asserted. In order to do this, he asserts the loyalty of Bedouin tribes and guarded the pilgrim caravans well, earning him trust from the Porte. He also worked together with the Ottoman military to defeat Abd al-Aziz ibn Saud of Najd and Muhammad Idrisi of Asir, both of whom were his rivals in the region.

With the political troubles in the centre from the spring of 1912 to the summer of 1913, the sharif enjoyed relative freedom of action in the region, and he managed to secure the election of his second son, Faysal, as deputy of Jidda, in addition to ‘Abdullah, who was reelected for Mecca. However, the new CUP government that established power in January 1913 heralded for the tightening of central authority. On 15 January 1914 the Porte appointed Vehip Paşa to the dual post of governor and commander of the forces in the Hedjaz. While the CUP valued the services of Husayn in restoring relative order to the region and furthering government influence in Arabia, the appointment of a high-ranking general to the combined post signified the intention to strengthen direct authority in the Hedjaz. This of course clashed with Husayn's intention to preserve his authority and evidently Vehip and Husayn disliked each other. Vehip continuously attacked and criticized Husayn's actions and decisions. He accused Husayn of misgovernment, arbitrary practices, and self-assumed privileges in the Hedjaz among many others. Vehip and Husayn also disagreed over priorities and jurisdiction. Vehip took issue with the Husayn's demand to accord top priority in construction of projects to those related to the pilgrimage. He also wanted to impose new taxes, to which Husayn would consent only if the proceeds entered the emirate’s treasury. He also accused Husayn of spreading slanderous rumors in order to have certain government officials removed in favor of his own men. The Porte continued to respond to Vehip’s reports by urging conciliation, advising that those matters will be dealt with in due time and prohibited Vehip to further provoke Husayn. Vehip however, continued to argue against the government’s conciliatory policies, insisting that they would fail. Finally, in July 1914 he advised "for the sake of Ottomanism" that Sharif Husayn should be dismissed and replaced by his predecessor, ‘Ali, for Husayn desired the downfall of the state. As Porte once again exhorted Vehip to get along with the sharif, Vehip concluded that either he should be transferred to another post or Sharif Husayn be dismissed, as friendly relations with the sharif were no longer possible.

With the outbreak of the war, Vehip found himself as the commander of the Hedjaz forces and Husayn found himself under pressure to endorse and promote the jihad but he refused to commit himself. Despite the early contacts from the British with Husayn, he's still reluctant to cooperate with Britain since cooperation with the Porte had been useful in fulfilling his personal ambitions in the Hedjaz. The possibility that the call for jihad might harm his popularity if he failed to endorse it, deepened his apprehension. Nevertheless, Husayn’s adoption of the jihad would have presented equally problematic prospects. Ottoman entry into the war rendered the Hedjaz particularly vulnerable to British aggression. Britain blockaded the Red Sea ports, leading to food shortages. The endorsement of the jihad would have ruled out any maneuvers to mitigate British reprisals against the Hedjaz. Even worse, the Red Sea coast was the most exposed region of imperial territories, while Ottoman commitment and ability to defend it was limited. Finally, an alliance with the British might have offered new and enhanced opportunities to Husayn for aggrandizing his power in Arabia. The straw that broke the camel's back was the discovery of a document belonging to Vehip's escort, revealed plans between Vehip and the Porte to depose Husayn and his family and to end the autonomy of the Hedjaz. When the disclosure was communicated to Husayn, he lost all hope of conciliation with the Porte and started looking towards Britain. In July 1915 Husayn sent a letter to the British high commissioner in Egypt, Sir Henry McMahon, setting forth the conditions that might persuade him to enter into an alliance with Britain and to launch a revolt against his own Ottoman government

Sources:

Arabs and The Young Turks by Hasan Kayalı

A History of the Modern Middle East 6th Edition by William Cleveland and Martin Burton

Turkey, Islam, Nationalism, and Modernity by Carter Findley