I suddenly have 10 2-year old running around, chasing butterflies, responding to nature's calls etc etc
But I do not know where they came from!!
There has been no Going indoors, and no 'taking care of baby' mothers.
Neither has any barrel of babies arrived prior to it.
Has anybody else experienced this phenomenon?

You don't, by any chance, have a male in your village who has breeding skills, do you? Many people have discovered, sometimes quite dramatically, that "trained" males will choose that particular activity, even when you have set them to do something else!

The most recent example can be seen in this post!

I did not have any males designated for breeding. I do check my villagers frequently. There had been no nursing mothers before the babies appeared. 10!! of them.
My lifedrive crashed once that morning, could that be the reason? It has crashed before with changing anything in the village.
Aliens?

Your LifeDrive crash probably had nothing to do with it. You don't have to have a given villager designated for breeding....they only have to know how (which, of course, only requires one successful "encounter")! A single male who "knows how" can run around the village making babies without you even realizing what's happening. That's why I always have my females initiate breeding - damage control! The males in my village remain totally without breeding skill!

When I play the game I have often missed events which require only an 'ok' rather than a choice. I wonder if you did the same here and just missed the barrel'o'babies?