This from fitwerx.com:
One concern with Di2 is that the $900 rear derailleur is a big deal to replace in the event of a crash. Shimano engineers realized this and, should the bike crash and strike the rear derailleur, it disengages the worm drive and moves inbound towards the wheel to protect itself. When the system is put under load again, after a few shifts it resets. Should the crash be bad enough that the automatic alignment does not reset to your liking, you can use the system adjustment buttons on the fly to adjust the shifting. This adjustment only takes seconds when practiced and it is actually easier than a standard cable adjustment.
So it seems that Shimano anticipated a shock side loading and properly engineered the rear derailleur shift mechanism to withstand some sort of side impact which, in the absence of a ‘breakaway safeguard’ would try to drive the worm gear via the ring gear which is mechanically not possible. This is a mysterious failure indeed. My engineer’s gut tells me this likely happened through some form of side loading during shipping…perhaps multiple times, bouncing in the plane cargo hold, continual loading from stuff piled on top of the bike…something along those lines. Murphy can be a real dickhead.