Did it say "Status Pending," not "expiring"? If so, I think it's for standbys who are checking bags.

I once stood by for the earlier SFO-NRT and had to check a bag. I recall some sort of "pending" tag in addition to my priority tag on my luggage because I didn't have a confirmed seat on the flight. Presumably it's so the baggage handlers know to only load my bag after I got a seat assignment.