What caught my eye was the bees that are not quite dead. My guess is that
they are getting into something toxic that is taking a little time to kill
the adults. This leaves time for them to get it to the hive, into the
stomach of the nurse bees, and then to the larvae. This could be a contact
poison that is coming in with the pollen, or something in the nectar. There
may be someone in your area that is using a pesticide on flowering plants
that the bees are finding.