I suspect that as well as your faction leader's ancilliaries and traits and how they effect his standing with the senate and the people, plus the effect of any conquered provinces and mission results of course, that there might be a random factor upon game start, adding or subtracting one or more points of standing.

The reason I believe there is a random factor is that while I was playing around with the campaign start file I once began a game where my leader was instantly ordered to commit suicide. I'm fairly certain that I hadn't changed the file between that and the previous game, at least not in an area that should have effected my leader's popularity. I didn't go back and check since I was busy testing other areas, but it's something worth looking at when I get home.

It's definitely possible to guarantee being outlawed at the start of the game. I began a Scipii campaign last night after giving my leader as many ancilliaries and traits I could that boosted his standing with the people and reduced his senate standing. Naturally I refused the invitation to commit suicide and so I'm already in a civil war with my fellow Romans, plus I still have to deal with the Carthaginians... should be amusing.