There was a rebellion in heaven
The motive for the rebellion is that Lucifer wanted some glory for himself. Lucifer's downfall was already scripted, but the "free will" of the Humans was unscripted, and Lucifer saw the opportunity to challenge God over the hearts & minds of men.
We're told in 2 Corinthians 4:4 that Satan is the "god of this world" which makes sense because its a doomed material world.
You are experiencing an individually crafted series of tests meant to reveal the content of your character and level of moral consciousness.