The causes were many and complex ranging from economy to plebs. But IMO the socio economic decay is the first cause. The first Imperator Octavian Augustus did not hesitate to send his own daughter in exile to Pandateria Island (modern day Ventotene) because of impure actions. Centuries later Imperators and state officials made amorality a standard of pride. As Ariel Durand stated, a great civilization is not conquered from without until it has destroyed itself from within.
excellent intervention friend, I agree with you, partly also I think that was related to jus gentium.