Who Sided with Haq ?

On 18 zilhaj prophet(peace be upon him ) clearly declared Ali (a.s) as his wali and successor and all the companions jointly and unanimously congratulated Ali as their Mola...but why a group of specific companions decided to breake this oath of allegiance. A question worth asking and worth exploring because the event happened at Skeefa completely changed the course of Islam and dissected it into two major factions ...Shia and Suni...and till date we are unable to bridge this yawning gulf of our beliefs and dogamas.