If you've been following the news, pre-order demand for the iPhone 7 has been exceeding all expectations. Originally most analysts believed demand for the iPhone 7 would be tepid because by all measures it's a marginal update over the 6s/6+. Then the day after the iPhone 7 was announced T-Mobile launched a free upgrade program that allowed iPhone 6 users to upgrade their phones to a 7 simply by turning their 6 in...along with committing to service for 2-years. This is the first time such a huge subsidy has been offered on a single phone purchase ever since subsidies were discontinued in the USA market (ironically by T-Mobile with their "uncarrier" promotion). On the same day T-Mobile announced the free upgrade, Verizon and AT&T followed as well.
It might just be that the carriers are using this promotion as way to compete and steal customers from each other, how they used to do before phone subsidies were stopped, and will eat the upgrade cost themselves. On the other hand, it might just be a sneaky way for Apple to get a bunch of these future-diseased iPhone 6's out of circulation, to allow them to avoid a massive recall. Apple kills two birds with one stone with this strategy - they take back the 6, which they can fix and resell into overseas markets that can't afford brand-new iPhones anyway and where Apple has been killed by lower-priced Android offerings - and they goose domestic demand for an otherwise-tepid release of the iPhone 7. The strategy may be working - Apple's stock price is up over 15% since T-Mobile and others announced the upgrade program.