There is no definitive answer to this question. We see some references in Scripture to a possible explanation:
How art thou fallen from heaven, O Lucifer, who didst rise in the morning? How art thou fallen to the earth, that didst wound the nations? And thou saidst in thy heart: I will ascend into heaven, I will exalt my throne above the stars of God, I will sit in the mountain of the covenant, in the sides of the north. I will ascend above the height of the clouds, I will be like the most High. But yet thou shalt be brought down to hell, into the depth of the pit (Isaiah 14:12-15).
But by the envy of the devil, death entered the world, and they who are allied with him experience it (Wisdom 2:24).
In Luke 10:18, Jesus uses the example of Satan falling from heaven as a warning against pride.
Based on these Scripture passages, various theologians throughout the centuries have woven together theories based on pride, envy, and refusal to worship. Ultimately, we do not know what caused Satan and other angels to rebel and be cast out of heaven. We can focus only on our own salvation.