Originally Posted by
tbessie
Ok, I just called United myself; they said even they don't usually know why this happens, but usually it's due to Economy getting full and UAL wanting to have more control over seating (probably so they can get more high-paying customers before then, I'd guess). They said if no Economy seats are available at checkin, they'll assign her to Economy Plus.
This has become far more common since they stopped Silver's from being able to select E+ seats at time of booking. Instead they get dumped in E-, filling it and leaving E+ largely empty.
At ~24 hours out a reasonable number of those Silvers will check-in and move up to E+, freeing many of the E- seats for your friend to be put into.
Your friend has two options - she can either try and grab an E- seat at around the 24 hour mark after a Silver has moved to E+ (in which case she'll likely get a Window/Aisle), or she can wait until the airport and potentially get something in E+ (likely a middle), but just as likely end up in a middle seat in E-.