Why does my dog keep having barking spasms

My family and I recently got a seven-week-old puppy, Cookie. She's a mixed breed, an Australian Shepherd and a Laborador Retriever. We love her a lot, but we can't seem to understand why she sometimes just suddenly bursts out in these barking spasms. Does anyone know what might be causing this?

It could be several things, anxiety, fear, over excitement, the reason why isn't as important as what you do when she has these sudden bursts. If you want them to stop, the correction is quite simple. When she breaks out into these barking fits, using your index and middle fingers (as if to simulate teeth) give a quick but firm touch to the side of the neck, followed by a "sshhtt" sound. The touch will redirect her attention away from whatever it is she's becoming fixed on and barking about and brings it back to you, and the noise lets her know that her behavior is unacceptable. It's as simple as that. If she continues to bark, again, give her the touch followed by the noise. She may then seem to become uneasy, or unsure of what she should do next, (largely because the behavior hasn't been corrected in the past) this would include her pacing around, whining, or her stiffening up and starting at an area and then glancing back at you. Any of these behaviors should be corrected again with the same touch & sound technique. Once she has settled herself into a calm and submissive state, give her a nice massage on the neck and back. This reassures her that her calm and submissive behavior is acceptable and is rewarded with affection, whereas anxious or obnoxious energy is not tollerated and repremanded.