Dogs usually wake up barking because they've usually heard an
unfamiliar noise, which made them wake up. It may have startled
them, too. Their bark is really a warning to let you know that they
think something's wrong.
A dog's hearing range is much wider than ours and they can hear
things in the higher frequencies where we aren't even aware there
even was a noise.
Then again, it could just be someone slamming a car door a few
doors down.
Dogs are protective of you and your family and it's just
watching out for you.