Thanks for being Johnny On the Spot with the link, BROJ.
Deviant, that thread has a lot of discussion on this very topic. To sum it up, the conclusion of that thread thus far is that the Pendant actually is what choose to stay behind. She didn't drop it, it wasn't some fluke of the transportation process, but a concious choice. This is concluded from other instances in the game where the pendant most certainly acts under its own power.
Of course, this isn't the only possible theory (and I am not sure if it can even be claimed to be the official stance of the Compendium). That is just what we have so far, so feel free to join in on the discussion.
As a side note, some people identify the conciousness behind the pendant as Doreen or the Entity. I think where we left off, such interpretations were possibly but not as strongly supported by the evidence.