Those dead bees are drones. Though not as long as a queen, they are fatter, so struggle to get through the queen excluder. I often find a few dead drones on the tops of my excluders when I have separated brood boxes with an excluder and drones hatch from the frames in the upper box. In their attempts to get out of the hive to fly and/or mate, they get stuck in the excluder and eventually die. As their bodies dessicate and disintegrate, the worker bees will remove them from the hive. It's not a symptom of disease or anything, it's just a result of having drones hatching above an excluder, where the hive has no top entrance.

A reason for separating brood boxes with an excluder could be to confine the queen to make it easier to find her when one comes to make splits. Another reason for having drones hatch above an excluder is one method of swarm control: namely checker boarding, i.e. removing frames from brood boxes and replacing them with empty comb, placing the original brood frames in the honey supers above the excluder. As the brood hatches above the excluder, there will inevitably be some drones get trapped above.

Do you notice drones flying out the top of the hive when you take off the roof?

These are drones in this picture, because of the large eyes which encompass most of their head - very large compared to a worker or a queen's eyes.