The breakdown of trust between the leaders of Shechem and Abimelech does not immediately erupt into open warfare. Instead, the conflict descends into secrecy and anarchy, marked by hidden traps stationed in the surrounding mountains.
The primary approach among commentators is that these mountain ambushes were set as personal traps for Abimelech, designed to capture and assassinate him if he traveled through the area [מצודת דוד, רלב״ג, מלבי״ם]. This hostility was also directed toward any Israelites who remained loyal to his rule [ביאור שטיינזלץ]. However, a different perspective suggests a more defensive motive. Because the leaders of Shechem were terrified of Abimelech, the men stationed in the mountains were not assassins, but rather lookouts meant to provide an early warning if the king approached [אברבנאל].
Regardless of their initial purpose, the men manning these posts soon abandoned their mission. Hidden away in the isolated hills and no longer afraid of the king's authority, the rebels took advantage of the growing lawlessness. By casting off the yoke of the government, they turned into common highway robbers, choosing to loot innocent travelers who passed by [מצודת דוד, רלב״ג, מלבי״ם]. Rather than organizing a disciplined and focused rebellion, the city of Shechem simply devolved into a community of thieves [ביאור שטיינזלץ].
This descent into banditry had a direct and critical consequence. Most commentators agree that the rampant highway robbery was exactly what exposed the treasonous plot to Abimelech. Hearing reports of the chaos and stolen goods, the king realized what was happening and carefully avoided traveling near Shechem [מצודת דוד, רלב״ג, מלבי״ם]. This unintended exposure of the conspiracy through the robbers was orchestrated by God to ensure that the guilty parties would ultimately face their punishment [מלבי״ם]. In contrast, a dissenting opinion argues that when Abimelech received the reports about the ambushes and the looting, he simply ignored the news and dismissed the threat entirely [אברבנאל].