For me it's due to getting E+ at t-24 as a UA silver. Then of course I keep checking if I didn't get my ideal seat in the hopes that someone with a good E+ seat received their upgrade.

Not sure if this is still the case but back in 2012 -2014 when I was 1K (and upgrades were actually a possibility), check-in time was a factor that impacted the upgrade list so that was a key reason - Wednesday afternoon we always took a pause and would all rush to try and check-in first (a bunch of consultants who were all 1Ks). A couple of times I got the upgrade over my boss because I was faster at checking in and got the last upgrade at the gate and he was stuck making a right turn (back when 757s frequented IAD - LAS)