This was more or less a quesiton on my final yesterday. One of the options was because he was consul and I think another was because he was pontifex or something, the rest I don't remember.
I know that becoming sacrosanct was from the taking of an oath. But the question seemed to imply that 'because he was x' meant he was sacrosanct.
One became sacrosanct by becoming Tribunus Plebis (Tribune of the Plebs). The Roman Senate granted Caesar tribunicia potestas (tribunician power) in 48 BCE, meaning he had the power of a tribune but was not an actual tribune because he was not of the plebian class; he was a patrician.