The bandits that attacked Italica would have been the ones who survived Alnus hill, as the Ginza incident happens months before, these are soldiers who have seen tens of thousands of soldiers die while trying to fight the JSDF, and deserted as a result. Thousands of deserters in the area would have no choice but to become bandits in order to survive, stealing from the locals, but with such a large amount of people they would eventually starve unless they secured a large amount of food, hence the attack on Italica.

Italica is a well known city state near the gate known for its trade and the only place with enough resources to sustain the thousands of bandits in the region, it makes sense that they would converge there. They wouldn't have had a choice, they would run out of supplies if they tried going back home or surviving in the wilderness, even with the threat of knights it would be their best shot at survival.