Over and over, if you do a quick review of the last 60 years of the Republic, there's this ominous sub-text of what the legions will do; and so many little choices were made over so many decades that developed the army into a decisive tool for one warlord or another, none of them seeming so dangerous at the time:

Marius, finding too few propertied Romans in the army, so throwing enlistment for the first time open to the 'head count.' Who, by definition, had no financial resources to go home to after service, hence the issues of land settlement become critical (i.e., no pension, but you get property in return for service)

Sulla, breaking 400 years of tradition by turning Republican armies on Rome and marching against the established government.

Pompey, who started the trend Caesar continued, of generous distributions of booty and money to his soldiers, so that they came to expect this from 'their' general

Again Pompey, who camped his legions so close to Rome that he intimidated the Senate into giving him the Consulship, even though he'd never served in any prior office;

Long periods when armies fought for one man (the "Special Commands" of Pompey and, perhaps most, Caesar's nearly 10 years in Gaul), when the soldiers could get to know, and become increasingly loyal, to 'their' general;

Caesar, marching HIS legions against the established government, just like Sulla, Lepidus, and others;

Octavian, who literally bribed away a portion of Antony's legions after the Ides of March and was paying so much in donatives to those men who would join him, that Antony's own legions mocked him for offering them less.

What seems so weird about this, when you consider the awful history of land-for-veterans during the Republic, how often it was fought bitterly (Cicero, for example), etc., is that - once Augustus got settled, one of his first steps to fix the problem was (a) to reduce the number of legions (b) to pay them regularly, so they didn't need either booty or a rich general to survive (c) to arrange for retirement by land settlement; and (d) to make them swear an oath to him, as Caesar, more important than any loyalty to a general officer leading them.

All of which could have been done in Republican times to solve the problem that led to the engorgement of the legions - but which no one did.

When Caesar's IX Legion (and later, the X and others) mutinied against him, it says worlds to me that the leaders in the army had learned that all the warlords needed them, and they could negotiate to sell their services to the highest bidder. Caesar never trusted either legion again - and right he was. They were now loyalest to their own interests.