Hello everyone
IMO the seeds of the Republics fall can be traced back to the Punic Wars.
Before the acquisition of Sardinia, Corsica, and Western Sicily conquered cities were generally given a good amount of self-rule, their main obligation to the Roman state was in soldiers. The Roman Republic was, in effect, a sort of federation. The territories acquired in the First Punic War were governed in a new way, directly by Roman magistrates who came to treat "The Provinces" as cash cows to be milked for their own personal profit during their term of office.
The 2nd Punic War ruined the Italian peasantry as a result of constant conscription keeping people from their farms as well as the devastation caused by Hannibal's invasion, a lot of peasants lost their land to aristocrats, who turned the land into slave-worked latifundia. The dispossessed peasants that migrated into the cities would come to support demagogues pushing land reform, enter the Gracchi. Since Rome's military was based on the conscription of those with enough property to afford millitary equipment (technically it was wealth, but property generally meant wealth in an agricultural society) there was a problem. Enter Marius.
It was the fatal combination of provinces ran by magistrates from Rome and the Marian Reforms that doomed the Republic. It was only a matter of time before a proconsul decided to conquer Rome for himself with his personal army.