First of all, there was no "year zero." The correct term for it is 1 BC, or 1 AD if that's what you're looking for.
Well, I can imagine sometime significant happened that made scholars start keeping track of years. Probably something religious, because that's all that scholars were back then; religious philosophers.
What I've been told to believe is that on year 1, Jesus was born, or something along those lines. Since I'm not really into that kind of stuff, I think it was actually the conception of the bible.