Because you gave specific flight info for the DL-insiders to help determine what happened. Since nothing was mentioned by one of the vocal DL insiders (see another similar thread here:
http://www.flyertalk.com/forum/delta...over-plat.html) as to "why" it happened, my guess is that maybe there was S....
Seriously? Your post is only 55 minutes after the OP. Give people a chance to look into it before adding fuel to the fire.
I've sat in someone's seat while boarding to talk to a traveling associate and then moved when the pax came along. It could be something as simple and innocent as that.