OHGODTHEY’RECOMING
I don’t actually know what was going on with these creepy decayed blowfly carcasses. If you haven’t noticed, that top one is missing its head and the bottom fly’s wings and eyes are flaking away. I found these and a half dozen more stuck fast to a sandy overhang by the Red River this past Entoblitz. It kind of looked like a still scene from Attack of the Zombie Flies.
Why were they congregated there? What killed them? Personally, I suspect a bacterial infection or pathogenic fungi. Many such parasites can actually alter the behavior of their hosts, causing them to seek out situations and habitats that help the pathogen grow better or infect new hosts. There’s even a little bit of evidence that pathogens may be able to manipulate human behavior: a few studies indicate that people infected with Toxoplasma gondii (often carried by cats) may be more risk-prone. The protozoan parasite may alter the behavior of its intermediate rat host to increase the chances of predation by cats, allowing the parasite to complete its life cycle (Berdoy et al 2000).
So, anyway, I’m going to go with mind-controlling fungal infections on this one.
That, or aliens.
Thoughts?
6legs2many 
Could it be a shed?
No, because these are adult flies, not maggots or pupae. The only insects to shed their skins after they have wings are mayflies, which have an unique subimago stage.