I haven't had specifically the same situation as yours, but many koi will bleed profusely from the gills when netted. I think it is a case of stress induced high blood pressure blowing a vein in the gills. I have had koi stroke out where mine would be going down the pond, and suddenly lose any sense of direction, crash into things and almost immediately, go belly up on the bottom. The netting may have stressed the koi into having extremely high blood pressure blowing out a vein and causing a stroke, but that is only a guess.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard