Cage was unclear about the logic here because Jimmy had some information involving Pat that he did not share. However, for Jimmy, it was already enough to compile this information together.
These people must be Frog's peers or other gangs coming to grab turf. Previously, when Nut was around, maybe due to combat power or other reasons, nobody had reassessed the distribution of territories.
Now that Frog had taken over Nut's territory, to others, this was the best time to expand their domains.
Jimmey stopped the drunk driver, and if brought back to the police station, the next step might be spending a few days in detention, which would seriously affect the hitmen's plans and could possibly blow their cover. So upon receiving a call from Jimmy, they came over, ready to hijack the driver back. These gunmen were not locals; as long as they dealt with the situation and ran off to other places, they wouldn't have to worry much about police retaliation.