Interesting behavior a few days ago.
Last week (Dec. 23), traveling with my wife and 3 yo daughter on an award ticket from my wife's account. Both wife and I are gold, daughter no status. None of us have cards with an award upgrade benefit - wife is most relevant since the miles were from her account - has a Select, as well as the new TravelBank card.
Checked in online at maybe t-7, and as expected, no option to split, after all, this is an award ticket and no benefits for upgrades. Checked status and no one at all on the list and I know we weren't split at check since the 3 BPs in passbook all had the same PNR still. Checked status again on the shuttle to the airport, and surprised to see when I checked the list that all 3 of us were on the upgrade list (the only ones) with 4 seats remaining (319). Needless to say I was shocked. Even weirder still, daughter (no status) was number 1, with wife 2 and me 3. Eventually, someone else goes on the list, but they go at number 4. At this point, Economy shows as full...I'm certain it wasn't this way before.
Just prior to boarding, someone else takes a seat in F on the map, I'm pretty sure not an upgrader since there are still 4 on the list, and no one additional showed as cleared (though I suppose could have been a buy up). Whatever happened, appears it was a UM, as she was sitting next to wife. So the three of us end up clearing into the 3 remaining seats.
I'm sure I'll never know for sure, but the only thing that makes sense to me, based on what happened, is that Y became full/overbooked, and there were no other elites in Y, except for perhaps the 1 additional on the list. E+, per seat map, only had a handful of seats selected initially (and then showed all selected when I saw economy listed as full - I'm assuming those went mostly to BE fares). So I'm thinking someone - either in IM or at the airport - saw we were the elites left that they could upgrade vs. op-up non-status in the chance it was oversold. In the end, apparently they wouldn't have needed it - as the flight went out with 6 empty seats in economy (E-/E+). But seems strange to me.
Also not the only time this year I've been upgraded on an award. In Jan, I was on a very short haul, also on a 319, that was practically empty, so that time wasn't a 'its full' situation. But it was on an award, I think F was also completely empty or was only 1/8. Surprised to get a CPU...I think 24 hours or so in advance.