Word I got from the SF people who I trained with around 1986ish was that someone had not been wearing their protective goggles while guiding in one of the helicopters for a landing. He got blasted in the face with rotorwash and instinctively used one arm, with the flashlight/signal cone thing on it, to cover his eyes. The pilot took that as a signal to move to he side abruptly due to some perceived obstacle, bumped into another parked aircraft and all hell broke loose.