As a cameraman who has also made docs, I would sometimes keep one eye in the camera and one eye open on the rest of the scene to be sure I was capturing everything intended. Following a strange noise, especially in documentary filmmaking, is completely normal. He also followed the other fireman's gaze. It was a lucky shot, a horrible shot, and an important shot that we needed to see, even though we didn't want to. It's an indelible moment caught on tape, impossible to fabricate, impossible to forget.