Bingo! UA851 yesterday ORD-ANC. Off at 12:37PM, arrived at ANC at 7:17PM. Looks like they're going to keep the same plane and have a special flight to PEK (UA1754, leaving today, ETD 8:40PM). Thankfully the vast majority of passengers are going to PEK as their final, so that helps from a rebooking standpoint.

Shows that the reason for the cancellation/diversion is "Airport Conditions" - maybe there was a freak storm in PEK that wasn't expected when they left ORD?