With all due respect, I'd much rather have them go quickly than linger on.
That said, chickens pick up many things and I wonder if they've eaten something that they've found. Someone mentioned "Tomcat" mouse things. Some of them have poison and while it may kill the mouse it may also kill anything that eats them as they lay dying.
Be sure to pick any nails or screws or other things laying around. I've seen an xray where the chicken ate a 1" screw.
Mine have picked at the plastic that I've covered the run with. Those drp clothes are out this year as they're to flimsy.
I've noticed my chickens pecking at the wood on the coop. Splinters may not go come out so well in their system.
Point being ranged birds are likely to pick up many things. It goes with the territory.
Not that it makes losing a bird any easier.