"Unexpected shutdowns" refers to situations where a device, such as a computer or a smartphone, suddenly powers off or stops working without warning. Full definition
The company said the update was designed to prevent unexpected shutdowns in devices with older batteries. (businessinsider.my)
About a year ago in iOS 10.2.1, we delivered a software update that improves power management during peak workloads to avoid unexpected shutdowns on iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, and iPhone SE. (businessinsider.com)
In a letter to customers this afternoon, Apple explained that it added a feature in iOS 10.1.2 that successfully reduced the occurrence of unexpected shutdowns on iPhones. (cultofmac.com)