Satan gets punished by God. God sends him to the 9th circle chained in hell for nine days. After he gets unchained and Satan realized he is not welcome back into Heaven. He decides to make a new leadership and rule Hell.
As far as I know, that's not explicitly specified in the Bible. However, it must have been an act of rebellion against God.
It could have been arrogance, trying to gain more power or to become god...
Milton's Depiction of Satan in Paradise Lost is Unique, because while he is definitely a "bad guy", he is sympathetic. We can understand why he makes some of the decisions he does, sympathize with...