I think it was the Huns, pure and simple. They were the worst, most frightening thing the Germanic peoples had ever seen. The Germanic tribes ran away, as fast and as far as they could, and the only place to run to was over western Europe, all the way to northern Africa. In this view of things, the short-lived empire started by Attila is the main story; the fall of the city of Rome and the western Empire is one of the side effects. The phrase "wrath of God" was originally coined to describe the Huns. The famous line of Conan's, when someone asks him what is best in life -- "To crush your enemies, to see them driven before you, and to hear the lamentation of their women." -- was originally attributed to Attila.
For a long time, historians have been pondering the question of why (the western provinces of) Rome fell. Perhaps ever since they fell. Malaria has been mentioned as the culprit. Lead poisoning. Christianity, because it made the Romans stupid and weak. (A theory I like very much for sentimental reasons, strongly advanced by Edward Gibbon,who is still in many respects the last word in the historiography of the Roman empire, and whom you should read now, NOW! if you haven't already, and to which I used to subscribe, but it doesn't seem to hold water.) All of these and other theories posit that the fall happened because of some weakness or decay within Rome. My theory, on the contrary, asserts that Rome was as strong as it ever had been, that the decisive change occurred among the Visigoths, Ostrogoths, Franks, Lombards and other tribes along the empire's border, or rather, on the other side of those tribes from Rome. I don't deny that the Dark Ages were dark in western Europe, chaotic and impoverished and miserable -- some historians do deny all of that, but that's a notion so thoroughly stupid that I've already wasted too many words on it -- but I don't see it as the culmination of a long and steady decline of the western empire, but rather as the result of a sudden cataclysm: wrath of God spreading out from central Asia, killing and burning all in its path, Germanic tribes running away in sheer terror, coincidentally overrunning the western Roman provinces, flowing over borders which had been more or less stable for centuries. According to this view, the earlier barbarian incursions, the weakening effect on the empire of increasing numbers of soldiers from barbarian backgrounds in the imperial military, corruption, impoverishment of all but a very few super-rich Romans and other symptoms of decadence would all have been greatly exaggerated in retrospect, in the constant effort to explain a weakening of the western empire which, say it with me everybody, didn't actually occur.
I could be wrong. It's just a theory. I want to be very clear about that. I don't think anyone knows for sure. I'm not at all sure why (the western part of) Rome fell. What I'm trying to do here is offer food for thought. Encouragement to examine some assumptions. And I'm certainly not the first person who's said: It was the Huns! It's very hard to know what happened in large part because a large part of the darkness of the Dark Ages consists of the very small amount of writing, historical and otherwise, which has survived, the illiteracy of the new ruling classes and the sudden obscurity of the literate classes.