This is a first time occurrence for me. I have two late July splits in the shade, partial shade anyway, that are bearding. I say bearding, but this isn't exactly what I thought bearding looks like. Anyway, it is usually said that they leave the hive because of being too hot. Yes it was a little warmer than it had been,mid 80's, but the real change was the past two days they have been bringing in the goldenrod nectar.
So is clustering outside of the hive not really heat related, at least in this case, but more of a function of drying down the nectar? Maybe the bees have to leave the comb to facilitate better air circulation.
By the way these hives do not have ScreenedBB's. My other hives in the yard, which were in full sun, do have screenedBB's and have no bearding.