Imagine you got to the airport in-time to check a bag ... (and that your checked bag was tagged onto your flight as planned)
But that due to Easychicken acting up and general mayhem at the counter you weren't able to checkin before your flight's departure ...
(The curbside checkin people declined checking you in because they weren't allowed to pull e-certs for your upgrade)

Who would you blame for the fiasco ? UA ? passenger ? both ?

would you expect to be charged a 100$ change fee to get on the next flight if you are told "you missed your flight" ?

Apparently the bag made it on your original flight , but not you ... isnt this a security violation of baggages rules ?

From my perspective : if UA accepted your bag on your flight then they should have been able to check you in for that flight .... (if you had been delayed through security, after checking in, then it would have been your problem, not UAs)