Of course we don't get cold hard numbers or reasons behind the closure, but at least Sony officially made a statement on the closure.
Sony Computer Entertainment can confirm the closure of Zipper Interactive, a Seattle-based internal game development studio that has been part of the global development operation of Sony Computer Entertainment Worldwide Studios (SCE WWS).
The statement goes on:
The closure is a result of a normal cycle of resource re-alignment within SCE WWS. Zipper has completed all work associated with its most recent project, Unit 13 for PlayStation Vita. Zipper titles MAG, SOCOM 4 and Unit 13 will continue to be supported, including the new Daily Challenges in Unit 13.