Did you put the bag under your seat? Even if you were sitting in the front row, that's a no-no. I don't see how a bag can move all by itself from the row in front of you to the row behind you before the plane starts moving. Southwest apparently doesn't believe that happened. Yet it would have made sense to ask everyone if they recognized the bag. If you were drugged asleep (Ambien is powerful) you might have appeared to respond and not recall what happened.
Dear Moderator NSX - this is totally unacceptable speculation. It is not a joke either even if the post is by a trol. Shame on you.