You get there, and it's all very underwhelming. The place is even filthier than Rome, sometimes little more than a village built with sticks. Food and weather are atrocious. The locals can't understand you, and you can't understand them without going through some local translator you may or may not trust. There is nothing interesting to do except adjudicating the occasional squabble between neighbouring tribes. You are too far from the action in Rome to keep up with the political winds, and you know people are forgetting you with every passing day. Your troops are bored and restless, corruption and moral decay are everywhere in your ranks. You're increasingly paranoid and fearful for your life - if one can be assassinated in Rome, where it's full of people at all times and eventual scrutiny is inevitable, think of what could happen here in the middle of nowhere, where nobody is watching...
So one day you snap out of it. You will be glorious, peasants be damned. You will be a Caesar, you will conquer and embiggen Rome's glory. What are these idiots going to do anyway, with their little wooden sticks? Let's remind them how we got here in the first place! And off you go into unprovoked conflict.
[0] https://en.wikipedia.org/wiki/Early_life_and_career_of_Juliu...
I was just riffing on what was a pretty standard story for governors and similar officials in far-away posts - precisely because Caesar's success in engineering favourable military campaigns established a sort of blueprint.
This seems to be a common cause of Roman rebellions. For example, Boudica rebelled after the Roman governor tried to seize her kingdom and then raped her daughters. The Jewish rebellion started after a Roman governor seized money from the Temple.