Both had married the daughters of caliph Abu Bakr which made them the brother in law of Ayesha. Ever since caliph Umar had appointed both as members of the Shura committee their ambition to become Khalifa had inflated beyond all other desires. Their influence, wealth and power had grown and they craved for the Khilafat.
When the ummah got fed up with caliph Usman and as frustrated groups of opposition began to emerge, it was Talha and Zubeir who rallied them and took charge as leaders of the mobs. Later they laid siege to the palace of caliph Usman and agitated matters to an extent which resulted in his murder. Everyone in Madina had witnessed their conduct towards caliph Usman and therefore contrary to the expectation of both, the people did not choose either of them as caliph. The public’s choice for caliph became Imam Ali. In order not to make themselves too conspicuous they gave their pledge of allegiance to Imam Ali.
Finding caliphate beyond their reach, they eyed Basra and Kufa as consolation prizes. Displaying obedience to the new caliph they waited in hope that their prestige and influence with the people of Basra and Kufa would not be overlooked. But Imam Ali was not impressed by their status and prestige. Imam Ali was reviving the true Islamic government and looked forward to appointing honest, sincere and God fearing governors in all provinces.
Neither Talha nor Zubeir possessed any of these lofty qualities. Both were unreliable. They had rebelled against caliph Usman previously and could just as easily rebel against Ali. And that is what they did! Realizing that they would not get Basra or Kufa, they decided to go to Makkah and left Madina with treason in their hearts. They broke the pledge of allegiance and soon declared war against the legal government in the name of vengeance for the very man they had gotten killed – caliph Usman!!