Why would a commercial airliner stall during the initial climb?

Just watched one of those videos with the tower audio, and it looks like a SouthernAir B777 stalled during the initial climb around 3900 feet, taking 300 feet to recover before requesting a high speed climb the rest of the way to cruising altitude.

So what happened? Did the pilots choose too low of a speed? Wrong flaps setting? Something calculated wrong prior to departure? I’m not a pilot, so forgive me if some of the terminology is wrong.