Well, there is no way to narrow it down to one specific event or condition, in history things are rarely ever (really rarely, I want to say never) attributable to a single action. However, these other people have done a better job than I can and looking at some of the factors involved.

/u/Iguana_on_a_stick answered How accurate is the popular view that "uncontrolled immigration led to the fall of the Roman Empire"?

/u/Tiako answered How "sudden" was the fall of Rome?

and /u/Polybios answered What were the consequences of Rome's conquests, and did it contribute to the downfall of the empire?