The debris does not seem to come from next to the failing rocket engine but from the outer wall next to the engine that was not used in the landing attempt. It appears first shortly before the clock shows 6:21.
Not sure why you got downvoted. When talking about 'explode on the pad' it usually refers a rocket blowing up right when the engines lights up before it lifts off.
Or alternatively during fuelling, SpaceX had that issue during Amos-6.
"I bet they needed that."
As others have noted, SN10 is standing by. Somewhat uncomfortably close by: https://i.imgur.com/F9rsBbD.png
It's so exciting to be a spectator of these events.