Are they under a light? Maybe the crane flies are attracted to the light reflecting off the beetle’s shiny carapace? Although you’d see them swarming the bulb too if that was the case.
A quick Google didn’t turn up anything about the species interacting. The only thing I could think of is some kind of scent, like scharf said. Some crane fly adults eat nectar, if that’s a nectar-eating species maybe the june beetles were previously on a plant that left some pollen scent on them. That seems like a stretch though, especially since most crane flies don’t eat at all in their adult phase.