RoughRomanMemes

Funny enough, for the first ~200 years of the Empire's existence, the Roman military, despite our occasional jokes about it, remained remarkably loyal at its core. There was only one real incident of civil war, and even that only broke out because of utter chaos at the capital making it unclear, constitutionally, where everything stood. Over the period of the Principate, there are numerous incidents wherein legionaries literally execute their own commanders for attempting treason against the Empire or attempting to bribe them to subvert their loyalty (and not for lack of love of money - legionaries loved getting bribed under ordinary circumstances).

The real issue starts in the 3rd century AD, as the legitimizing functions of the central government weaken. Ruthless Emperors destroy the position of the Senate, which, while a fundamentally oligarchic body, is the last real connection with a legitimate claim to representing the 'people' of Rome. Furthermore, as Roman coloniae operate with the same government system as the old republican system, but in miniature, the perception of people is not "Central council of aged bootlickers slowly stripped of its power" (which is closer to the truth), but "Last elected body of the Empire sidelined" (which is further from the truth, but more intuitive to a people who had widespread old-style local senates). At that point, what is the incentive to stay loyal to the central government? Perhaps loyal to Rome, still, but who reigns in Rome becomes a much more 'fluid' question of loyalties. A collapse of the pseudodemocratic 'imperial ideology' of 'popular consent' which bound ordinary people to the dictates of the central government.

Funny enough, the misidentification of the army as the core problem is one of the weakening factors of the Late Empire. After seeing legionaries no longer loyal to the central government overthrow it numerous times, the Emperor Diocletian in the late-3rd century AD decided that the true problem was not the central government, but the army itself, and massively reformed it. He also reformed the central government, but to be even more autocratic. (Some of the reforms involved were of the mid-3rd century AD and thus predated Diocletian, but many are attributed to Diocletian, and also fuck Diocletian, so he gets the blame here)

The army then became little more than a divided group of armed thugs, very far from the professional legions of old, though still very effective in combat. While the Legions of old were largely not 'nice' people, and prone to various abuses, there was also a robust system of unit pride and loyalty, as well as discipline, which made legionaries not just brutal, but also capable of steady and unglamorous work, and occasionally garnering praise from civilians for being capable of peaceful conflict resolution and suppression of banditry, as well as the aforementioned loyalty to the central government.

Unsurprisingly from this analysis, the army continued to support civil wars against the central government to replace "Their Autocrat(tm)" with "Our Autocrat(tm)", as that and dynastic loyalties had become the main legitimizing functions of the central government - not any conception of the broader consent of the people which ordinary recruits might feel some amount of hesitance to disrespect. So instead of solving the issue, the army remained just as troublesome, but much less useful to the people and to the central government, exacerbating the issues of the Late Empire without actually alleviating any problems.

tl;dr; the core of the problem was that the central government lost what little connection it had to the people, and the people are who made up the army