Probably a production ramp.
Assuming Apple is announcing the next-generation iPhone that day and shipping about 9-10 days later, they need their contract manufacturers to accumulate enough inventory to support the launch (which will undoubtedly be limited to a handful of countries and supply will be tight anyhow).
The production ramp for an iPhone launch is probably about six weeks.
A secondary issue is likely the finalization of iOS 6. The iOS engineering team need to wrap up version 6.0.0 of the software which will be flashed on the finished hardware.