Random baby zombie in adventure mode survival

I doubt anyone could reproduce this, but it was just another day, exploring the wilderness. I went into a nearby cave and there was a zombie that was trying to attack me and I thought "no big deal". But then I saw the strangest thing in the history of my experience of Minecraft. I saw A RANDOM BABY ZOMBIE COME OUT OF NOWHERE FROM THAT ADULT ZOMBIE.I have no idea what to make of this. I am telling the honest truth, though I was too surprised from the whole ordeal I forgot to take a picture. Sorry.