1. It is impossible to really know what happened without insider information

2. Cancelled flights and same day changes are the biggest factor in last minute changes to the upgrade list.

3. The comment: "The folks in the front row, well, let's just say they didn't match the average profile of DMs.. " is really kind of ridiculous. So many people fly regularly and you just don't know. The "young couple who seemed a little enamoured with their 'first class' seats" could have been one DM and one general member who got a companion upgrade and were excited about, for example.

4. It is good practice to confirm that first class has boarded full (not checked in full) if you are at a place on the list where it looks like you will just miss the upgrade. If a pax who has an F seat no-shows, you could actually get it, but some GA's will not come on board to move you if you have already boarded, they will just go down the upgrade list until they find someone who is still in the gate area.

It is possible this happened with your first flight, that for some reason several seats were held (maybe for a group that was misconnecting or VIPs or something) and then released again when they did not show, and the GA was running out of time and so just upgraded people who were in the gate area to get the flight out. On time departure generally takes precedent over everything, and if there were pax with unassigned seats and coach was already full, it is possible that this happened to avoid taking the time to go on board, move 5 pax up and then reassign those seats to new people.