IT WAS ALL THE SACK, ONCE IT WAS SACKED IT WAS ALL OVER 😭
For those who are curious about the actual details, the Western Roman Empire (the fall of the Empire as we generally think of it) fell around 476 AD, when the last Emperor was replaced by a Germanic warlord. But it was only marginally functional by then - Majorian (died in 461 AD) was the last real independent ruler, and Rome had been sacked twice (once in 410 AD, once in 455 AD). And even the sacks were not really a cause so much as an effect.
Rome went through a period in the 3rd century AD known as the Crisis of the Third Century, wherein the old institutions of the Empire as most of us imagine it were broken and brutalized by increasingly naked military rule, plague, economic crisis, barbarian invasions, constant civil wars, and a breakdown of government legitimacy. By 284 AD, with the ascent of the Emperor Diocletian, these issues were compounded by a curious mixture of centralized power and decentralized authority, necessitating increased taxes and repressive oversight from officials, including a form of proto-serfdom for the poor.
Over the course of the 4th century AD, the Empire's fortunes deteriorated in every conceivable aspect, and when the Empire was split between Arcadius (Eastern Empire) and Honorius (Western Empire) in 395, the Western Empire was very clearly on its last legs, all-but-broke, unable to muster enthusiasm from its own population for its defense, and unwilling or unable to curtail the power of its elites even for the sake of self-preservation of the polity.
At that point, one is basically asking for the barbarians to sack the place for the nice narrative ribbon to tie around the decline. Especially with the sack in 410 AD - caused by the incompetent Emperor Honorius murdering his best general and best troops for the crime of being (checks notes) loyal while Germanic, leading many of them to join the Germanic tribes in opposition to Rome - who had agreed to not attack in exchange for a bribe that Honorius also welched on.