Thanks for the different perspectives. I did take a glance at the app as I was boarding (early as 1K) and F still looked full so it must've been T-15 as to when it got cleared out. As someone who flies weekly and looks for flights at least every other day, the late surge to fill out F did feel a little odd, especially as they subsequently failed to show. My theory is that the removal of change fees leads people to book a lot of flights just based on the positive optionality of having no downside if they choose to not get on it, which is why I think this type of thing could reoccur. The weather could've played a part but I felt like something more than that just based upon my past experience and ORD that day. I did respond to the followup flight email United sends to atleast let them know how it occurred. Going forward, I'm going to just try and stick with a sure thing looking for PZ inventory than trying to play high odds otherwise.