They DO NOT know "when it happens".
There are video cameras and there are high-speed film cameras. The film cameras have to be offloaded and the film processed at the lab. Then the engineers have to look at EACH one FRAME BY FRAME (picture by picture for the laymen out there).
When I worked there as a engineering/documentary photographer, I counted over 120 cameras we had on one launch alone.
Film cameras run anywhere from 96 frames per second to over 400 frames per second. The long range tracking telescopes usually had film cameras running at 96 fps (at least when I worked there)(They also had video cameras but your're not going to get the "motion-stopping" ability with standard video which runs at 30fps or 60 fields per second).
The debris event in this mission occured about 70 seconds into the flight. That means that an engineer had to look through 6912 frames of film from just ONE CAMERA to get to this event. There are several long range tracking sites at the Cape.
Launch pad cameras cannot be accessed for several hours after launch until the pad has been safed. Then it takes another several hours to access all the cameras in their blast-proof housings, download the film and take it to the labs. It takes hours to process the film.
It's NOT instantaneous folks.
That's nice, but where are all the tracking cameras to film re-entry? There are none. NASA is relying on amateur video.....ridiculous.