Mine have been doing that over the past few days (possibly they do it every day but I am not always able to be here to watch).....
There seems to be 1/2 hour/hour in the afternoon when it gets really busy..... lots of very purposeful bees darting off, lots coming home, lots apparently "just" milling around..... but on closer inspection they look to be just "queuing" to get in..... plus quite a few coming out and circling/spiraling (before presumably setting off to forage).....
It seems to only be for the 1/2-1hour - the rest of the day they just appear "busy" - fairly constant stream in and out without congestion.....