The bible doesn't say when the angels of Gen 6 were cast down to hell and locked up in chains....
That particular sin of all those marriages between the "sons of God" (fallen angels) and the daughters of men did not take place simultaneously. Therefore those angels could not all have fallen in the same moment of time.
Some of the angels committed other sins than this one of marriages with human women. From the cautionary warning of Paul against the voluntary "worship of angels" in Colossians 2:18, who are "greater in power and might" than humanity (2 Peter 2:11), it would appear that once a righteous angel decided to accept the worship of men, that they fell into sin by that means. Satan seemed to be most desirous of getting Christ to fall down and worship himself in Luke 4:7, so this must have been one of the temptations that righteous angels could have succumbed to.
This worship of devils is mentioned several places in scripture, particularly by the Gentiles. As Paul once wrote in 1 Corinthians 10:20, "But I say, that the things which the Gentiles sacrifice,
they sacrifice to devils, and not to God..." And also the text of Psalms 96:5 (LXX), "For all the gods of the heathen
are devils, but the Lord made the heavens."
Israel also fell into this same sinful practice, as Deuteronomy 32:16-17 said when Israel departed from God and sank into idolatry. "They provoked Him to jealousy with strange gods, with abominations provoked they Him to anger.
They sacrificed unto devils, not to God; to gods whom they knew not, to
new gods that came newly up, whom your fathers feared not."
This phrase about
"new gods that
came newly up" shows that the righteous angels fell into this temptation to receive the worship of men over the process of time, and became devils on a case-by-case basis, instead of a massive, one time only unified revolt against their Maker.