For any non pilots in here: stalling is not ideal. Stalling at 4kft over a city center is super not ideal.

How did an A320 stall? AFIK half their systems have to break for that to be possible...

It didn't stall, obviously, or else the headline would be different. The aircraft alerted the pilots with a stall warning twice and it's suspected that there was something faulty with the sensors of the aircraft.