For the record, this is a case ubiquitous in every empire, as whenever an empire declines its stability is affected.
When we search for the reasons of the Roman Empire , here are a few:
High inflation: High inflation was something that brought down the strength of the Empire a considerable amount. Around and about the 3rd century, the administration introduced a currency known as the denarius, which is just a silver coin. Now, of course the difference between inflation here and modern inflation is that the over valuating of the currency in the modern era is due to the amount printed, during the old Roman times, since it is made out of precious metal, it is a different concept. When you offer a coin made out of a pure substance (bronze, silver, gold), it maintains its face value, the way they managed to then devalue the currency is when they failed to meet the supply and demand of the currency (for expenses). Then, the government halves the currency in order to pay their debts, which is no problem, but whenever someone gets the coins, since merchants and the such are aware of the low value of the currency, they charge higher rates. It goes on and on into this death spiral of inflation.
Barbarian Nomads: Before the Barbarian Invasions started, hordes of itinerant barbarians started to settle in the outskirts of Rome, they essentially gained a foothold on the Empire. This was the case in the Eastern Roman Empire, when the Emperor Valens allowed the Goths to settle south of the Danube (fleeing Hunnic persecution). Rome, subsequently had a famine, and they sequestered when the Goths asked for the land and/or food that they were promised; whilst also being abused by the Romans (such as by exchanging food for slaves). This led the Goths to depredate the Romans for around 6 years, wherein in 378 they obliterated the Roman army at Adrianople (https://en.wikipedia.org/wiki/Battle_of_Adrianople); Rome lost her most knowledgeable tacticians, and Emperor Valens at the battle supposedly burned alive at a farmhouse, an apocryphal statement circulated by the Goths.
High Expansionism, High Upkeep: As you very well may already know, the Roman Empire expanded to unprecedented heights, being one of the most vast empires in history. You do have to keep in mind that since their economic model is not based on Capitalism or anything of the sort, their economics were different, this goes hand in hand with describing them as unsophisticated when it comes to modern terms (of course they were the most advanced in their time). Therefore, everything was less effective, it is estimated that the average American is 74 times as economically productive as the average Roman. Duncan-Jones estimates that the amount of gold spent on the Roman army (just the army, not navy) in 150 A.D. is around 16,800 pounds of gold, my point is that the Roman Empire was incredibly overextended, lots of unassimilated cultures, and had a tight budget, which is all conspicuously did not ameliorate the circumstances of the Empire.
Religious Rift: When Christianity was promulgating around the Empire, there was a great division starting to form within the very ranks of the Empire. It could perhaps be described as a veritable Great Wall, where there were Catholic Romans, and Greco-Roman believers. This was most prevalent in the ranks of the army, where both sides had extreme animosity towards each other, and were not cohesive force, which also aided the downfall of the Empire.
Last image is from the Battle of Munda (45 B.C.) by the way. https://en.wikipedia.org/wiki/Battle_of_Munda