I think what you are seeing is being driven by a couple of factors.
First, as eBook use is growing exponentially, so is the use of the library Overdrive system. Some libraries who were fairly liberal in the past about borrowing rights to their digital catalogs are starting to tighten down just because they simply cannot serve an expanded client base. The waitlists are getting too long and their funding isn't growing to match the demand.
Second, Overdrive itself is starting to restrict certain books to libraries that do not carefully limit borrowing rightly only to the set library district boundaries. This is because of pressure by the Publishers. Because of this, some libraries that previously allowed access to reciprocal or out of district clients have discontinued that policy.
You can read about that here.
http://librarianinblack.net/libraria...overdrive.html