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!