I have known this to happen with tropical fish (for aquariums) although very rare. I suspect that the stress of jumping out of the tank or falling out of a net onto the floor along with the impact is enough for some fish to call it quits. More of a stress/shock related death rather than physical trauma.

Is it possible that the fish were merely knocked out?

I have tossed a few pan fish pack into ponds that have died over my lifetime, but always contributed the death to deep hooking or the stress of being caught.

Fish on!,
Noel