Ibn Saud was present long before WWI but he became a key actor, to an extent, in the region after he was able to capture Najd (Riyad) in 1902. His supporters rallied around him and later he attempted to establish relations with the British however, they were reluctant given that they were afraid such relations would damage their connections with the Ottoman Empire. After the Ottomans declared war on the Entente powers in 1914, Britain saw importance in establishing contact with Ibn Saud and after a set of negotiations signed the Anglo-Saudi agreement in 1915. The treaty mainly declared Britain's recognition of Ibn Saud as a ruler over the Najd area and declared these lands a British Protectorate. The agreement also stipulated that Ibn Saud shall refrain from intervening in the affairs of other British protectorates near his territory including Kuwait, Bahrain, Qatari Sheikhs regions and the Oman coast, however, the agreement did not mention Hijaz which was westwards of Ibn Saud's Najd. On the other hand, With the onset of the WWI Britain was certain that its ties with the Ottomans are far over, and potentially viewed relations with Sharif Hussein as important given that he is a prominent Muslim figure and that he is a member of the Hashimite Clan of Quarish, a descendant of the prophet, custodian of the holy site of Mecca and Medina and above all ruler of the Hijaz. They offered him to be king of an Arab Kingdom in return for helping with driving the Ottomans out of the Hijaz. This agreement was constituted through the Hussein-McMahon correspondence that took place between 1915-1916. On Britain's side there were more agreements to be brokered first in the Sykes-Picot agreement with France (some historians argue that Russia are also an active actor in the deal and that it took place subject to their acceptance) and the Balfour declaration which declared that Britain viewed with favor the establishment of 'National Home for the Jewish People.' This just goes to show the amount of pledges and promises Brokered by the British during that time period where many of them conflicted with one another.

All these Promises came back to haunt Britain after the War with tensions between France, the Hashemites, Palestinians and Jews. Obviously, these agreements overlapped, and many territories were promised to more than one party. During the war, Ibn Saud tried to unite the Arab rulers in the Arabian Peninsula but each ruler had his own aspirations and course of actions which ultimately resulted in failure of unity. Ibn Saud also met with Emir Abdallah son of Sharif Hussein but an agreement could not be reached. While Ibn Saud negotiated the Anglo-Saudi agreement he was also approached by the Turks for a possible alliance but he turned them down saying that he could not oppose the British which makes sense given that they are the strongest force in the region. During the great Arab revolt Britain tried to reconcile Ibn Saud and Sharif Hussein and even pressured Ibn Saud to join forces with him however, Ibn Saud was reluctant to actually do so. Ibn Saud mainly contested Hussein's claim to being 'the King of the Arabs' and demanded negotiations with him to discuss the Hijaz-Najd border to which Hussein replied audaciously. After the war, in the year 1919 Hussein wanted to capture the city of al-Khurma after losing it in a struggle between two notables where one declared his control over it. Sharif Hussein's troops were decimated by the notable and some soldiers sent by Ibn Saud. Sharif Hussein later sent an 8000 man detachment lead by his son Abdallah to capture Turaba, an oasis near to Khurma. Notables supporting Ibn Saud rallied nearly 4000 troops and attacked Abdallah's forces at night after they saw the aftermath of Turaba and defeated them. Ibn Saud later arrived to al-Khurma with a big number of reinforcements (the actual number is not specifically known, but pro Ibn Saud sources estimate them to be 12,000 which is viewed as exaggerated) where he received an ultimatum by the British demanding his return to Najd citing that they would consider their agreement void and act upon that if he does not reatreat and allow Sharif Husayn to define the frontier. He did in fact retreat but this act showed the power and influence Ibn Saud was capable of.

By 1923 there was apparent discomfort in Hijaz with the pilgrimage tax and increased Zakat which Hussein used to strengthen his army. Many of the tribes of Hijaz who were not approving of the methods of Hussein fled to Najd. Meanwhile, tensions grew between Hussein and Britain after he voiced his protests more than once since the promises he was given by Britain never fully capitalized. He declined many further agreements with Britain to emphasize this notion. (His son Faisal ruled Iraq, Abdallah ruled Transjordan, Syria was controlled by the French and Britain took the mandate over Palestine) After the fall of Ottoman Empire in 1922, the Turkish republican government abolished the caliphate which prompted Hussein to declare himself the Caliph of all Muslims. This moved provoked Ibn Saud and aggravated Hussein's relations with Britain who feared that they would lose control over him and the Hijaz. The Issue was exacerbated after Hussein opened a soviet consulate-general in Jidda and recognized the USSR.

Coming to your question, Ibn Saud thought, now that Britain and Sharif Hussein had a turbulent relationship, they would remain neutral in case of a conflict between Najd and the Hejaz. He decided to raid the city of al-Taif to test British reaction. His forces entered the city and the the town's munitions depot was seized. Forces that tried to hinder the Najdi advance towards Mecca were decimated and hope for Hussein appeared to be non-existent. Nobility and Ulama of Hijaz decided to try and pacify Ibn Saud by forcing Hussein to abdicate in favor of his son Ali, which would negate the premise of war given that they saw the conflict residing between Ibn Saud and Sharif Hussein himself. Sharif Hussein was later forced to leave Hijaz and moved to Amman (Transjordan was ruled by his son, Abdallah) where he died in 1933. This did not stop Ibn Saud, in fact he maintained the march towards Mecca where Ali was forced to retreat to Jidda. In Mid-October of 1924 Ibn Saud's forces entered Mecca while he was in Najd. He decided to remain there in order to blame the nomadic fighting tribes which constituted Ibn Saud's fighting force if Britain was to object on his campaign. Ali was forced to abdicate after negotiations between Ibn Saud and Hejazi notables. Foreign consuls in Jidda declared their neutrality and demanded the safety of their nationals and their property. In late October Ibn Saud headed to Mecca and on December 13, 1924 he declared his programme for the Hijaz where he stratified his discourse in that the Hijaz will settle 'through consultations among all Muslims', that the 'legal base of the Hijaz will be the Quarn" and that he his chief concern is purifying the holy land of enemies of the faith namely Hussein while that he would allow the Ulama and those who serve the holy sites to remain in the Hijaz.

Sources:

The History of Saudi Arabia by Alexei Vassiliev

Saudi British Relations, 1939-1953 by Shafi aldamer

The Origin of British-Saudi relations by Jacob Goldberg