I'm not familiar with Malaga airport, but I presume the passengers were boarding via an airbridge? If so, what happened to the member of staff that backed the airbridge away from the plane so that it could push-back? Surely they would have walked back up the tunnel to the gate and seen 29 passengers standing there? Wouldn't they have notified someone to make sure the plane didn't take off?
Yeah they were boarding via Airbridge at Gate B12 (sometimes it's gate B13 as gates B12 and B13 are just two boarding card checkers right next to each other but both can be used at one as the other passengers get bussed) The airbridges however at Piers B and C are not see-through so they are harder to notice (Gate C31 and Pier D are).
Was there a chance maybe that they were waiting between the airbridge and the Gate as Malaga has a section where you have to go down the slope before you get to the actual airbridge.
Here in this video it shows the point once you have shown your passport & boarding card. This is Gate B17 which is two gates to the right of Saturday Night's Incident (I've never left from B before but I have arrived there and all the gates In Piers B and C are similar and do this):