In the grand scheme of existence, our universe may in fact be very, very small and in that context it's no great leap to imagine that if one universe could spawn, why not more?
The concept of "infinity" is a hard one for humans to grasp. In our minds, there has to be an "end" or a boundry because that is our environment on Earth. Our "universe" is basically what we can see or infer from our instruments. If the universe is infinite, the other "universes" maybe just be a continuation of our universe but farther away. Or not... if we ever find the wall and a sign "you are now at the end of the universe, go no farther" then we'll know.