Why do my villagers on minecraft keep multiplying?

So I spawned one in the house then it went to two then it went to three and while I was working on the basement of the house I heard more sounds when I went upstairs there was atleast 10 wtf just happened?

They must be bored! Give them something to do?!

Villagers will mate depending on the number of valid doors in a village. I'm not really sure how Minecraft denotes the boundaries of the size of a village. But if you have a lot of houses near you, the villagers will start mating and multiply.