In one word, the Goths. Everyone has their own opinion, mine is that it mostly comes down to population.

The population of the Roman Empire decline due to plague, while the Gothic population and culture had grown with better agricultural technology. When the Huns pushed vast numbers of Goths into the weakened Roman empire in the west they further weakened it. Italy starved, and they lost the ability to raise effective armies. Rome itself eventually fell to siege and starvation.

On the exterior, peer powers like the Parthian and Sassanid Empire in the East. On the interior, successful military leaders who tried to become emperor, weakening the Empire from within, which then allowed foreign powers to invade (though some of them were invited as foederati)

The author also talked about that in another post [1], though it's not the focus of it (mostly in the 'Kicking, Gouging and Screaming' part).

[1] https://acoup.blog/2021/07/30/collections-the-queens-latin-o...