I wonder if some activity outside the hive may get the bees in a defensive mode. Am thinking a nosy animal investigating the hive, may set them off. Here we come a while later, and not knowing what had happened, walk into a stirred up bee yard? Our two hives are generally pretty calm. Every once in a while I can walk down to investigate, or walking the dogs after they get fed their supper, I can tell that they are stirred up as I get closer.
Only once, I was sitting in my chair next to the entrance watching the bees, have I gotten stung. One flew from behind and bit me right off. I got out of there quick. Before that happened, they were just carrying on as usual. If I have the hive open and get stung, it's usually because I did something reaching into the deep hive that gets them mad. Am still learning.