Most of us have heard about “fallen angels” even though the Bible does not literally support it. If you research “fallen” you will only find one reference to an angel in Is.14:12, which is a prophecy of something to come. This scripture refers the “the Shining One, the Morning Star.”
“How you have fallen from heaven,
O star of the morning, son of the dawn! NASU
(The KJV adds the name Lucifer, but there is no such name in the most correct manuscripts).
The reason this verse is still future is because Rev. 12:9 is still future.
9 And the great dragon was thrown down, the serpent of old who is called the devil and Satan, who deceives the whole world; he was thrown down to the earth, and his angels were thrown down with him.
“The fallen angels” refers to those mentioned in Jude 6 and 2Pe.2:4.
And angels–those who did not keep the position originally assigned to them, but deserted their own proper abode–He reserves in everlasting bonds, in darkness, in preparation for the judgement of the great day. Weymouth. This seems to be a free will decision to abandon (desert) their own dwelling place and because they did,
2 Peter 2:4
For God did not spare angels when they had sinned, but hurling them down to Tartarus consigned them to caves of darkness, keeping them in readiness for judgement. Weymouth
Because of their own actions, God cast them out of Heaven; they didn’t fall, they were thrown out!
What sin did they did commit to cause this? For those who may not know we can find that answer in Genesis.
Now it came about, when men began to multiply on the face of the land, and daughters were born to them, 2 that the sons of God (angels) saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose. 3 Then the Lord said, “My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years.” 4 The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown. NASU
4 The Nephilim were on the earth in those days and also later. That was when the sons of God had sexual relations with the daughters of human beings. These women gave birth to children, who became famous and were the mighty warriors of long ago. NCV
The offspring of these angels were/are the Nephilim. Goliath was a Nephilim, a member of a super race of people, continually evil, mighty warriors and famous men of renown. Nimrod was another. This is the sin that these particular angels committed. They have been in Tartarus ever since the Flood.