A few of our villagers turned into zombies randomly, there was no attack on them. When we went to look for them we found 3 of our 4 villagers had turned. We had them locked away from any harm yet they transformed into *+regular zombies +*rather than zombie villagers. Is this a bug or intended feature?

Our world was set in easy. It is also a realm. This is the first time this has happened to us.
Any help will be appreciated!

Cheers