This theory is speculative, but let's give CNN's Don Sears some consideration anyway. He suggests that Apple's suspected iPad mini event has been moved from the rumored October 10 to 17 date range to the current expectation of the October 23. Why, he asks? Because the Microsoft Surface is due to arrive right around then, he answers.
Microsoft announced its Surface tablet unit quite a while ago, and rumors suggest that the device will greet the public this month. Sears suggests that Apple re-scheduled its own event to more closely coincide with Microsoft's launch plans. While much of this is based on suspicions and rumor (Apple has not officially announced an event), Sears' logic rings true.
After all, Apple has had no problem with stealing thunder. It has announced its own products specifically around big industry conferences like CES and GDC many times before, and it would be very Apple-like to take Microsoft's release on directly, stealing much of the anticipated Surface buzz. We'll have to see how this all plays out when these announcements finally do happen later on this month.