And that's important on those aircraft because unlike most other aeroplanes, the door slide does NOT disarm when opened from the outside. Hence, its the only means of those outside the aircraft being able to check if the door is actually disarmed or whether they're likely to get a slide in their face if they persist.
I believe that on a 737 the slide has to actually fall quite some distance on release from the door bustle (should it be opened in the armed mode) before the inflation system is activated. This distance is purposefully greater than the distance between the door bustle and the platform surface of the aero bridge or steps or catering truck from where the ground staff are opening the door that way in theory the slide should not inflate but just flop out and most probably giving them a hell of a fright. Now if the flight attendant bent down and pulled the red webbing loop, it would suggest to me they perhaps had a beef with the ground staff over the assignment of a delay