The leaders of Rome were becoming very unlikable and did not please the people of Rome.
Many emperors were assassinated within a year of ruling. This lead to Rome having a less stable government because of the absence of a strong leader.
An earthquake and tidal wave hit the Roman Empire, causing mass destruction.
These natural disasters resulted in a large decrease in population... which could lead to a weaker/smaller military or less government officials. It also decreased the economy.
Rome began to get weaker, and therefore become invaded by other civilizations.
For example, The Huns was a group that first invaded Rome. However, Roman people were somewhat happy, being relieved of a corrupt, weak, and falling government. They had no strong leaders. Justice was no longer being served in their empire. The end of the Roman Empire was inevitable at that point.