What’s going on? United blocking available windows/aisles in economy during booking — have to call to book them

I’m currently silver, 700K lifetime United miles.

Booked a flight SFO to EWR — 787-8/9 using the website. Flying regular economy. When I went to seat selection, only middle seats were available to book but the small pop up of open seats showed many aisles/windows. I booked the middle seat and had my assistant call United. United told her they are “blocking preferred seats” and my assistant had to convince them to unblock one of those for me to get me a window seat, which they did, but it sounded like it required a sob story.

I have never heard of this before or seen it before. Is there something new going on? I obviously want to avoid middle seats so it seems annoying to now add this additional step. Even as lowly silver I feel like I should be able to book any open seat… just tell me what it’s going to cost to do so!