The notion that Caesar sought to extend his imperium (or uphold its original length) to avoid prosecution is rejected outright by Gruen (The Last Generation of the Roman Republic, 495), and he notes that this narrative, originating already in antiquity, has found its way into virtually all discussions of the topic. Yes, the inimicī of Caesar had charges ready to level against Caesar, and had been clutching them since 58. They included (vaguely) accusations about procedural violations during Caesar's consulship of 59, which certainly occurred. They also probably included the famous "war crimes" charges of Cato. Gruen argues, however, that a fear of becoming a privatus (ie losing the legal protections of imperium) was never Caesar's prime motivation in the events leading up to January of 49. The sticking point, as both Caesar's own words, as well as Cicero's letters make (relatively) plain, was Caesar's right to stand for the consulship in absentia for 50, later pushed to 49. I strongly agree with Gruen that this had little to do with the threat of some petty trial back in Rome, and everything to do with Caesar's plans for the organization of Gaul and his own dignitas vis-à-vis the factio back in Rome and especially Pompey. As Gruen states, "lesser men" had easily skated away from such prosecutions many times in the past.

As to the first reason: Caesar had spent years fighting for control in Gaul, and as many again stamping out persistent and dangerous opposition to that control. Though it is never spelled out as such, it follows that Caesar sought an extension of his proconsular powers in Gaul in order to organize his conquests into a proper provincia, as countless others had done in similar circumstances (including Sulla and Pompey in the East in recent decades). Caesar seemed already to be thinking about such efforts upon the eruption of the conflict with Vercingetorix. No one doubts that the next step in Caesar's political journey was a second consulship back in Rome, and one motivation for that magistracy must have been the expedited implementation of his arrangements in Gaul. He had himself, during his consulship back in 59, provided similar services for Pompey's Eastern arrangements.

As to the second reason: again, I think Gruen is spot on:

if Caesar's offers for mutual disarmament were genuine, he was willing even to forego the ratio absentis in the later months of 50. But he would not yield submissively to the blusterings of the conservatives, much less to the bullying of Pompey. In the end it was a matter of personal dignitas. That was Caesar's own explanation for the invasion of Italy. And we have no reason to doubt it. (496)