#3 OVEREXPANSION It's hard for a empire to keep control when it is spread out too big. Rome kept conquering others and then kept their land which made it hard to keep rule over such a large expansion.
#2 RULING INSTABILITY Many rulers were in power in very short period of time which made the citizens confused and made them not trust the rulers. Rome rulers changed often during the civil war and without the trust of their people they faced many issues.
#1 BAD ROMAN MILITARY After the roman army became full of non loyal fighters, they had trouble meeting their needs of money which showed weakened their army and empire as a whole with no proper defense and empire.