I made a villager trading hall and one time I entered the place at night I noticed that about half of the villagers were taking damage and died. After this I found few drowned and lots of zombie villagers in a nearby river.

A bit later this happened again but this time I managed to wake up and thus save all but one villager. I checked the river again and found one drowned two normal zombies and a zombie villager.

So for some reason my villagers seem to die while they sleep and wake up as zombie villager twenty or so blocks north.