Why does this simple program result in "puppies puppies puppies" to the console?

This weird thing happened, and I managed to condense it down to just this small program:

int main()
    int a[20];
    a[21] = 5;
    return 0;

this program results in puppies puppies puppies continuously scrolling by on the terminal until I hit control-C. What's going on?!

Edit: What is unclear about this question? Please tell me so I can improve it.