So a few things, 1) how many available F seats and how many available C+ gates were there at the gate? 2) When the F upgrades clear, that likely opened a few C+ seats. If other people on the upgrade list boarded ahead of you, that drops them off the upgrade list. If you boarded in Main 1, it is conceivable that you moved up the list enough to take an available C+ upgrade.
Were you on an MD90, C series or 717? Many medallions won’t take the middle seat C+ upgrade.
(sorry, saw STL, not a C Series destination)
The gate agent may have also needed your seat to put a family together and moved you up.