OP will undoubtedly get some comp from UA as did I ($250 voucher) for what had hallmarks of turning into similar. In early 2015, I showed up at gate 5 minutes before boarding was scheduled to begin for BOS-ORD UX flight. All boarding was already complete, and they were paging me. There was no weather situation. I was pissed as I knew I wouldn't be able to fit my carry-on. GA was very rude and denied any wrong-doing. I find UX undisciplined and unreliable. I generally show up early for UX but just how early do you need to show up to deal with their shenanigans?