The company recently identified the battery as causing the problem and instituted a replacement program.
A Message from Apple about iPhone and Unexpected Shutdowns
We care deeply about our customers and take seriously concerns they have about their Apple products. After hearing reports from iPhone customers whose devices unexpectedly shut down, we thoroughly looked into these reports, and collected and analyzed devices.
We found that a small number of iPhone 6s devices made in September and October 2015 contained a battery component that was exposed to controlled ambient air longer than it should have been before being assembled into battery packs. As a result, these batteries degrade faster than a normal battery and cause unexpected shutdowns to occur. It's important to note, this is not a safety issue.
To help our customers who are experiencing this issue, we are replacing batteries in affected devices, free of charge. iPhone 6s owners can easily find out if their device is eligible by typing in their device’s serial number on the iPhone 6s Program for Unexpected Shutdown Issues web page. We apologize for any inconvenience this has caused customers.
We also want our customers to know that an iPhone is actually designed to shut down automatically under certain conditions, such as extremely cold temperature. To an iPhone user, some of those shutdowns might seem unexpected, but they are designed to protect the device’s electronics from low voltage.
We looked for any other factors that could cause an iPhone to shut down unexpectedly. After intensive investigations, no new factors have been identified. We will continue to monitor and analyze customer reports.
We encourage any customer who experiences an issue with an Apple product to visit an Apple Store or an Apple Authorized Service Provider, or to contact Apple Support.
You can check if your device qualifies for a free battery replacement by clicking here.