T-Mobile employees are apparently being blocked out from taking vacations from September 20th to September 22nd. Plus, rumors that the iPhone 5S would go on sale September 20th date back to the beginning of July. It seems more than likely that these two are linked together with purpose.
This report comes in fresh from TmoNews by unnamed sources who presumably want to remain that way. T-Mobile won't confirm or deny the rumor, but it's far from uncommon that carrier or retail store vacation blackouts point directly toward the launch of a new iPhone. Last year, both AT&T and Verizon prevented vacations during the week of the iPhone 5 launch. T-Mobile only recently started offering the iPhone, which explains why no such vacation blackout for the carrier was leaked with the others.
Don't forget, this year there supposedly won't be just one, but two new iPhones. The iPhone 5S and the lower-end iPhone 5C are both expected to make their grand and highly anticipated debut at an Apple event rumored to take place on September 10th. Two new iPhones could very well mean bigger sales, bigger crowds, and therefore higher demand for employees.
Samsung is also most likely going to unveil the Galaxy Note III on September 4th, but it's highly doubtful that this is the cause of the vacation blackouts. Samsung's device is timed nicely though for those who would prefer not to have to deal with the long lines the iPhone tends to cause.