Well, maybe some background information from my campaign would explain it.

The Sweboz first attacked me from the alps. I was allied with the Averni. They were getting rapidly exterminated by the Aedui and Sweboz. I intervened: destroyed the Aedui, drove the Sweboz out of Gaul and gave the Averni most of the Gallic provinces. The motivation was that I really didn't want to conquer Gaul in an early stage for myself; I wanted to be able to face the Gauls in late game.
I also took those two cities in the Alps (the two directly north of Italy) from the Sweboz so that I had a buffer against them, consisting of Type IV client states. Because I had beefed up the Averni considerably by now, and becaus I had launched a couple of punitive raids against the Sweboz, they never became much of a threat again. They still attacked my two Alp cities regularly but rarely with full stacks. Usually I could decimate a besieging army in one turn with mostly slingers. It's an annoyance to be sure, but nothing major.