Review of the tower instructions will tell whether American was told to hold short of the runway while Delta was taking off.
Or, worse, if Delta took off without clearance .
https://freerepublic.com/focus/f-chat/4123838/posts
Most likely cause discussed on this thread and you can watch the ATC Live video that shows positions of the planes. Barring something we don’t know about, the taxiing aircraft appears to have taken instruction for a different aircraft number and/or missed their turn and the sign that said runway.
AA screwed up big time .
AA went on a Diversity pilot hire binge 3 years ago .