I don't think you'll find your experience was a one-off...
Back in February, attempted to fly Vegas to LAX with American. Was on Standby, so we were held back till last minute at check-in, until we had seats allocated. Queues through security were ridiculous - wouldn't allow us priority. Had to run to the gate...to watch aeroplane pushing back at STD+3 minutes...with our baggage on board.
They left 30, yes 30 pax behind......
Quote: "...but we're an on-time airline sir..."
Had to hire a car and drive to LAX to collect the bags...
Wonder no more, why AMR is losing money when they leave behind 25% of their revenue pax - let alone the security implications of going with unaccompanied bags..