How do angels become fallen angels?

Angels in heaven became proud and greedy for power perhaps and became fallen angels like Lucifer.

Answer:

There's no term in the Bible that says "fallen angels."

Neither does the Bible use the phrase: "the fall of man."

These are coined by men, probably derived from a scripture or two that they have read and expanded upon and embellished in their minds.

Scriptures like where Jesus says: "...I beheld Satan as lightning fall from heaven." (Luke 10:18)

No doubt, the angels that followed Satan "fell" also with him... but, "fallen angels" is still not in the Bible.

Peter speaks of: "...the angels that sinned..." (II Peter 2:4).

Jude talks about: "...angels which kept not their first estate, but left their own habitation..." (Jude 1:6)

According to Peter, then... the "fall" of these angels was due to the fact that they "sinned." [sin is the transgression of the law - I John 3:4]

They "broke the Ten Commandments"... the universal laws of God.

It began with Satan lusting after "God's job" as the Sovereign Authority over the entire universe.

"...for thou hast said in thine heart, I will ascend into into heaven, I will exalt my throne above the stars of God: I will sit also upon the Mount of the Congregation, in the sides of the north: I will ascend above the heights of the clouds; I will be like the Most High." (Isa.14:13-14)

So if "falling" is a valid term for man to use... then those angels men refer to as "fallen angels" achieved that status by "sinning." By "breaking the laws of God."

Breaking the law is sin, no matter who does it... angels or men.