Does it say anywhere why there are so many fey and other First World monsters in the Stolen Lands? I think one of the books says something about the barrier between planes being thinner than most places, but I don't think it ever explains why this is the case.
No, nor does it explain why the BBEG is so interested in it. I think it's just supposed to be an anomaly, one of those places where the prime is touched by another plane.
*** spoilers for the AP! ***
In my campaign, I had the Stolen Lands be the place where Count Ranalc and Nyrissa courted and kept their relationship secret for a time, away from the intrigues of the faerie courts of the Eldest. Their visits opened the way to other faeries, and the place's significance to Nyrissa's past provides some rationale as to why she wants to bottle it up.
