Why did Julius Caesar fear prosecution when his term as governor of Cisalpine/Transalpine Gaul?

Fantastic question! It's been a while since I've answered one, so you just made my day here. Let's get into the whirlpool that's 1st century BCE Roman politics, shall we? :) The first posts will be background to help you understand the last post. If you just want a quickie, scroll down a bit to the third post. If not, enjoy!

First thing you have to remember is that Rome in the last century BCE was confusing as hell. Power was constantly shifting, and traditions were being overridden left and right. In the early years, Roman armies had marched on Rome for the first time - and then there were the actual civil wars. One of the few traditions that was still left was the eternal tradition of politics. Every man wanted more power - and in that regard, it was every man for himself. There were no political parties, but there were political "ideals." You could appeal to the people for your power - these people were disparagingly called the "populares" by the other side. The other side was traditional and focused on looking as "Roman" as possible - they called themselves the "optimates" or the "best men." Many pop history folks (looking at you, Dan Carlin) like to portray these two sides as the "Democrats and Republicans" of the ancient world, but that's absolutely not the case. Every man was for himself - and that was the greatest check on the system. While you made temporary alliances with others, you would only help that other person out so long as it was convenient for you. Backstabbing was hugely prevalent, and it was also the greatest check on any one man having too much power. Men rose and fell with incredible rapidity - Caius Marius, for example, was a paragon in 101 BCE. In 100 BCE, he was panned and forced to retire from his 6th consulship in (relative) disgrace due to the Saturninas affair, which sparked a whole new set of SNAFUs. But that's another story, and this is just for the addition of context.

Now for just a bit more context. This might seem random, but bear with me - I'll tie it together in just a bit. In 60 BCE, there was a rather defined political battle between a few of the biggest names in Rome.

The first, possibly the most common, name was Cato Minor, more commonly known as Cato the Younger. Cato was one of the most outspoken men of the Senate; he never held the consulship (Rome's highest office), but he certainly did command quite a bit of auctoritas and dignitas. When he spoke, people listened - and he was famed for not budging from his firm ideals on what a Roman SHOULD do. That didn't really work out in his favour, other than being the most famously outspoken man in the Senate at that point, but his reputation garnered him one crucial advantage; people were afraid to speak against Cato. Cato was the staunch defender of Roman ideals! Obviously, if you speak against Cato, you're speaking against Rome herself! You're not a true Roman! You get the idea. Cato's famous for holding the first recorded filibusters (I believe), and he was nothing if not diligent at curbing people's power. Needless to say, this policy didn't go over terribly well with those who were seeking that power, which leads us to the first person who Cato royally pissed off!

Pompey Magnus. In 60 BCE, Pompey was the most decorated Roman general alive. He had been one of Sulla's understudies, earning his title of "Magnus" in the Marian civil wars. After that mess, he spent a few years getting his ass handed to him by Sertorius in Spain (which is generally glossed over rather conveniently), for which he was granted a triumph. From there, he headed to Italy, where he took credit for crushing Spartacus' revolt, then took command of the Mediterranean to conduct what was possibly the greatest anti-piracy campaign in history. Within 40 days, the entire Mediterranean was cleared of piracy - which is a bigger deal than it sounded. Rome's population was incredible, and the city was absolutely reliant on constant imports of grain from her provinces - something that the pirates were interrupting, on top of ransacking other trading ships and even raiding the coastline from Gibraltar to Egypt to Italy and Greece. Putting it in perspective....clearing that entire area in 40 days? Today, it would be considered to be a nearly impossible task. It's one of the achievements of Pompey that shows that he truly was an organizational genius. Immediately afterwards, he conducted the Third Mithridatic War, which culminated in the Pontic king's suicide and the subjugation of the Eastern Provinces.

When Pompey returned to Rome, he received another triumph for his success, and he was again the golden boy of the people. He expected all of that love and adoration to translate into political success, but he failed to reckon with Cato's....Cato-ness. When he returned to Rome, Pompey had two objectives: He wanted to give land to all of his veterans (a reasonable request, which had become tradition over the past half century), and he wanted to confirm his Eastern Settlement (the laws and regulations that he had established over in those aforementioned provinces). Cato cockblocked both with the help of his allies. Pompey, feeling rather stung by this, began searching for allies of his own. The only man who held as much power as him, however, was one of his own greatest enemies - a man who he couldn't stand. In 60 BCE, however, they both had a common enemy in Cato. And as they say, an enemy of my enemy...

Marcus Licinius Crassus - he's well known as the wealthiest man in Rome for a reason, and that's what he's most famous for. What people don't think of is what he did with all of that money. Especially in the First Century, a bid for political office required a vast sum of wealth; and most of the people running were running so they could achieve that vast sum of wealth. Well, they were really running for the power and prestige. The wealth was just a nice side effect - but they still didn't have the money on hand to campaign. They had to get it from somewhere, and the best place to get the money? From people who had the money on hand. Crassus was the most famous of these, and he bankrolled HUGE numbers of aspiring politicians. As a result, he had an enormous amount of sway in the Roman political system - all he was lacking was a military command. He tried to achieve military glory in the Spartacus War, but, as it was a slave revolt, that didn't do much for him (coupled with Pompey's glory-hogging tendencies). Well, in 60 BCE, Crassus was having similar issues to Pompey; Cato was being a nuisance. Crassus was hugely involved with the publicani - who were made famous in the Bible as the "publicans," or the "tax collectors" - and the publicani had a problem. They weren't able to collect as much money as they'd promised in the war-ravaged East (what a shocker). So, being businessmen and not wishing to make a loss, they tried to renegotiate their contract. Some of the Senate (such as Cicero), saw the demand as completely outrageous - but were in favour of it to placate the business class and to keep them mollified. Some, like Crassus, were businessmen themselves, and were wholly in favour of the idea. But then, in comes Cato and his cadre.

Cicero, always being the purveyor of wonderful descriptions of daily politics, commented thusly:

The fact remains that with all his patriotism and integrity he is sometimes a political liability. He speaks in the Senate as though he were living in Plato's Republic rather than Romulus' cesspool. What could be fairer than jurors who take bribes should themselves be brought to trial? Cato moved accordingly, and the Senate agreed. Result, the Knights declare war against the House - not upon me, for I was against it. Could anything be more shameful than tax-farmers repudiating their contract? All the same, the loss was worth standing to keep the Order on our side.