I played mostly eastern factions so my experience is limited on this issue.
I would guess that you're just a bit too successful and they thought, better just get rid of him in the beginning?
Playing the Brutii, however, I remember that the missions were very easy to accomplish and I always was done with them in under the time. I didn't expand that fast, though, so that might be the point. I think when you have too many popularity points with the plebs that it might trigger the civil war.
Anyway, all of the above is mere speculation.

R'as