This also happened to me. It started a few months ago.
I first got about 60 free diamonds and thought it was a gift, so I carried on playing (I never disabled the background task btw)
About a month later, I got 230 free diamonds and carried on thinking it was the same thing as before. The game got alot more difficult and
I stopped fighting people as I waited to level up and get even better warriors, however, this morning I opened the app and got the message
telling me that I had successfully purchased 159,247,096 diamonds (and I never used real money on the game!).
I am using a Lumia 820, I have no friends as neighbors in the game. I really wonder what is the cause of this...