Lucifer, who was the most spended of angels, did not like the plans God had of creating man out of dust: that Mary(the mother of Jesus) would have a higher place in heaven than the angels and that God would be born of a virgin and assume our lowly nature. Lucifer and his pride couldn't accept the humility of God. So, thinking he was better than God, Lucifer rebelled against his Maker.
St. Michael and the holy angels fought Lucifer and his unholy angels, casting them into hell.
St. Michael, defend us!