Xode, in Los Angeles, where I practice and where this case was filed, cases get dismissed without prejudice in one of two ways, when the plaintiff (here Prosper) submits a request for dismissal without prejudice, or if plaintiff fails to appear at a status conference, then fails again to appear at an order to show cause hearing. Typically, in limited civil cases in Los Angeles are on the slow track.
Since the case was dismissed less than a month after the proof of service was filed, and no hearing dates had been set, I'm completely confident that Prosper requested the dismissal.
I agree that the question is, why?