In Lucan's Pharsalia, do the shortcomings/flaws of Caesar and Pompey cause the civil war, or is it an inevitable outcome of the growth of the Roman Empire?
Lucan's Pharsalia is an epic poem, and thus tends to offer explanations based on personal motivations rather than more complex economic and sociological factors.
The opening books of the poem actually offer up a third possibility. They discuss at some length the various portents, prodigies, and auspices foretelling the war, and then speculate about whether the gods simply set in motion all things at the beginning of time to lead humans down an unchangeable path (to a degree mirroring the Stoic notion of necessity) or whether events and fortune are random (the Epicurean swerve).
The major cause assigned by Lucan for the civil war, though, was Caesar's pride and restless energy, abetted by Pompey's age, weakness, and ineffectiveness. Caesar, though carrying much of the blame, is not alone in causing the war. Crassus and Julia had acted as forces for peace, and their deaths contributed to the existing tensions evolving into full blown war. Curio and other supporters urge Caesar to take action, and the Senate's support for Pompey strengthens Pompey's resolve.
The crossing of the Rubicon by Caesar functions in the epic as a form of hamartia, a single decision that sets the rest of the action of the poem on an inevitable course. Once Caesar has crossed the Rubicon, he cannot turn back from civil war.
It is important to remember that this is a poem, not an extended prose history such as Thucydides or Tacitus, and even less like a modern work of scholarship, and thus presents the war in terms of character and action. Most modern historians trace the causes of the civil war to some of the inherent problems in the structure of the Roman Empire under the Republic, but that sort of account is rather too dry to make epic poetry; instead Lucan takes Virgil as both a model and rival, focusing on people rather than ideas.