1- `Umar's and his followers attacked the house of Imam Ali (a.s.) and set it on fire. This was done in order to force Imam Ali and his adherents to give allegiance to Abu Bakr.
2- Abu Bakr's and `Umar's refusal to give back to Fatima her inheritance (from her father).
3- Also because she considered Ali as the legal successor and vicegerent of the Holy Prophet (s.a.w.).
Historians have narrated that, when `Umar ibn al-Khattab brought some wood in order to set Imam Ali's (a.s.) house on fire, he was told that Fatima, the daughter of the Holy Prophet (s.a.w.), is inside the house too! He replied: LET IT BE!!! Then he kindled the fire and began to set fire to the house of the daughter of the Holy Prophet! "O son of Khattab! Are you really going to set our house on fire?" Fatima (a.s.) asked. `Umar replied, "Sure! Unless you agree on what the whole ummah had agreed upon." The daughter of the Holy Prophet (s.a.w.) said, "I do not remember a people that have done worse than yours... You have left the Holy Prophet's body lying among us, while, at the same time, you were busy in electing a caliph among yourselves, without consulting us or asking our opinion. You have plainly misappropriated our right."
Then `Umar and muhajirin proceeded in burning the house!
Al-Ya`qoobi, in his book, had recorded the following: "There were a bunch of people who attacked the house. They broke Ali's sword, and pulled out Ali with all the force they could muster and drove him toward Abu Bakr. Then Fatima cried `O Father! O the Apostle of Allah! see what we are facing because of the son of Ibn al-Khattab and Ibn Abi Quhafah(Abu Bakr) after your demise.' A warm argumentation occurred between Ali and those who were attending the meeting in the presence of Abu Bakr, `Umar, and Abu `Ubaydah. Ali tried to explain to the people his right to be the caliphate and his deserving of this position, but that didn't please `Umar who then began to threaten Ali if the latter refused to give allegiance to Abu Bakr, and said to Ali: "You have no choice but to give allegiance!" "And what if I refused to give allegiance?"
Ali asked. `Umar replied roughly, "I swear by Allah, the only god, that we shall behead you immediately!" Ali said, "Then you are going to kill whom he is the servant of Allah and the brother of the Apostle of Allah?" `Umar angrily answered, "It is right that you are the servant of Allah, but it is not true that you are the brother of the Apostle of Allah." After this dialogue, Imam Ali then left the meeting and walked toward the tomb of the Holy Prophet (s.a.w.).
There, he began to cry and speak to the tomb saying, "Son of my mother! Surely the people reckoned me weak and had well-nigh slain me!"
Regarding the issue that Abu Bakr and `Umar had refused to give back to Fatima her inheritance, one can refer to various historical sources and find more details.
When Fatima (a.s.) was ill for the last time before her demise, Abu Bakr and `Umar visited her. She seized the opportunity and spoke to them, "I shall inform you a hadith of the Holy Prophet, but, shall you be honest enough and tell me whether you have heard it from the Holy Prophet or not?" "Sure we will!" They both answered.
Then Fatima (a.s.) said,"I administer an oath to you to say the truth.
Didn't you hear the Holy Prophet (s.a.w.) say that, "Fatima's satisfaction is my satisfaction, and if she becomes angry I will become angry too. He who seek Fatima's love, is as one who seeks my love; and he who satisfies Fatima, it is as if he has satisfied me. Whosoever makes Fatima angry shall make me angry too." Abu Bakr and `Umar said, "For sure we have heard this hadith from the Apostle of Allah." "Then I bear witness that you two have made me angry and did not satisfy me. When I meet the Prophet (s.a.w.), I shall complain to him against both of you." Fatima (a.s.) continued. Here, Abu Bakr said, "I seek refuge in Allah against His anger and your anger, Fatima!" Then he began to cry until his soul was about to come out of his body. Fatima (a.s.) continued, saying, "By Allah I shall curse you in every prayer I may perform!"
Here, our question is: What is your opinion in this regard, to the manner of the two Shaykhs toward the Household of the Holy Prophet (s.a.w.)?Wasn't it more humane for Abu Bakr and `Umar to satisfy the Household of the Holy Prophet (s.a.w.), especially during and after loosing their (the Household's) Great Father, the Holy Prophet (s.a.w.)? Would it not have been more honorable if Abu Bakr and `Umar tried to solve the suspended issues between them and Ali (a.s.) and Fatima (a.s.) and at the same time they could have keep the respect of the Holy Prophet(s.a.w.) and guarded the benefit of the Islamic Ummah?