Thread for backyard hens but I have a question I'd like an opinion about

I had 3 older hens, and last week I got 3 young (2 or less year old) hens and put them in my coop. They seemed to get along pretty well except for a small scuffle between one of the new hens (picrel) and the most fit older hen.

I just expanded their 'free range' ability from a small 30x20 ft makeshift fence surrounding the chickencoop to a proper 4ft t-post fence giving them ~0.25 acres of land to roam, so they could get an even better diet and wouldnt be forced to butt heads. Fence was completed this past weekend.

Yesterday after coming home from running errands for less than 2 hours, i went out back to check on the hens. There is a small strip between my shed and a 6ft wooden fence that i think i heard a quick flutter from a flapping wing and there one of my new hens was, dead on the ground.

I inspected the area, there was no blood and very minimal feather scattering. Maybe a dozen feathers? The body was still very warm. I checked under the shed and didnt see anything.

What could have killed it? Was the flapping the hen having a heart attack?

Rip picrel, whom i didnt even name. Its the only pic of them i have