Sunnism
Commencing with the reign of Ali, the Shiites eventually got to taste the power they'd longed for. But it would not last. The Kharijites first, and then the Umayyads persisted as a thorn on their side. The Shiite leadership were eventually overcome by the Umayyads, who cemented their dynasty after upon laying to waste Hussein, his family and much of the Shiites.
The Umayyad who were relatively secularists did not need religious justification for their flagrant corruption. What they needed was a narrative to counter the widespread Shiite narrative of Imamah that still posed a political threat, even after the tragedy of Karbala. So, Abu Hurayra was employed to demean Ali, while the Shiites exalted him; to endorse the Ummayad while the Shiites denounced them. And thus, Sunnism was conceived. Sunnism is therefore a reaction to Shiism, not a precursor to Shiism.
By the end of the Umayyad reign, fabricated hadith had become propaganda by both the warring Sunnite Umayyad and Shiites factions. In cooperation with the Abbasids, the Shiites succeeded in toppling the Umayyads after nearly a century. But on attaining power, the Abbasids immediately ditched the Shiites, all their doctrines, all their bargains, and declared a Sunnite caliphate.
The Shiite had been fooled. The Abbasid recommenced Hadith fabrication and commenced its documentation. They could not go back on their promise to establish an Islamic state, so hadith fabrication became a means to qualify all their tyranny and transgressions as Islam. The Umayyads had conceived Sunnism, while the Abbasid shaped and fleshed it.