Short Answer: Because Nothing is unstable.

Longer answer: The vacuum of space is a sea of virtual particles that come into and go out of existence all the time. Occasionally a region of space some of these particles happen to be in undergoes "hyperinflation," increasing in size by 28 orders of magnitude in a second or so. A big-bang has occurred and a new universe is born.

According to astrophysicists, not only does this occur, but it has to occur. Think of it as the Principle of Cosmic Asymmetry. It explains why matter predominates over antimatter in our universe (when inflation occurred, matter went thisaway, antimatter went thataway). We even observe it happening, not on the scale of universes, but in black holes in our universe. This is what takes place: a particle-antiparticle pair appears--from nowhere--just outside the black hole. Normally they would quickly annihilate each other, disappearing from the universe. Instead, the antiparticle falls into the black hole (from which it can't escape) and the particle gets away. We observe this as a stream of radiation emmanating from the black hole.

Our universe is analogous to that stream of radiation. It is merely the result of the quantum indeterminacy of space.