I've benefited from operational upgrades for:
A.) overbooked in coach and they needed seats so they start with 1Ks and work back until they have enough seats for coach. One time I was already sitting in my coach seat and the gate agent came on board and asked me to move up to first.
B.) The gate or ticket counter agent decides I have experienced some delay, like the time I couldn't check my bags at the curb because my name was pulled for a random security check, so the very nice agent upgrades me for my trouble I guess.

I've had this happen as Premier, Prem. Exec and 1K.