In 58 BCE, Julius Caesar is having a blitzkrieg in Gaul, where he has essentially neutered one of the most powerful tribes of the Kelt and smashed the player from the outside who'd come in and intervened in Gallic politics. This has happened in just one year, with three battles, killing a lot of civilians, and Caesar has never been far from the political game, giving favors as he goes.
Tom Holland explains how Julius Caesar's winter quarters in the middle of Gaul sent a signal that he was there to stay, and how the Gauls saw this as the beginning of their conquest.
In a letter from Labienus, Caesar was informed that all the Belgae, located north of the area he was campaigning, were hatching a plot against the Roman people. Caesar reasoned that since they were working together against him, it justified him going after them.
Caesar indicts certain Gauls for inciting the Belgae to conspire against the Romans to seize regal power and talks about an anti-Roman insurgency in Gaul.