Another possibility is that since iPhone 5 production is ramping up, Apple is making the assumption that for the holiday season, a certain number of people will buy iPhones instead of iPads (or both). I could see iPhone cannibalizing a certain number of iPad sales for the holiday season.

Secondarily, it could be the case that certain hardware parts are shared by both the iPhone 5 and the iPad 2, and so if they're making those parts at capacity, shifting some of them to the iPhone will create a backlog in iPad 2 capacity.