People have identified over fifty different things which may have helped cause the empire to decline. From hyper-inflation, caused by massive imports but low exports, to lead poisoning, the failing of democratic accoutability, to plagues. And maybe that it simply grew itself to death, the bigger and more diverse it became the more it became filled with people who cared nothing for it, but far more for their local leaders and it failed to propagandize itself, well enough to make its thousands of new members care about it.

That last is an interesting one, because of course Rome was the first major republican empire. Not an empire made by one wave of conquest led by a single king, but one which grew by stages, adopting the ruling classes of other nations as citizens, as often as not. Thereby making conquest easier, (many people even asked to be absorbed ), but becoming ever more culturally diverse and also, ever more divided against itself. That may seem to be an anti-liberal, anti-multicultural, message, but it is only so because the Romans also failed in their education/propaganda needed to explain the empire to its diverse subjects, and it also failed to reward people by preventing a deepening rich poor divide. (Yes that may sound familiar.)

Three centuries ago, Gibbon was famously fond of the theory that Christianity was the cause, but while Christianity may have contributed a little, most people would agree today that gIbbons view was far too simplistic.