"Random shutdowns" refers to unexpected and unplanned events when an electronic device suddenly turns off or stops functioning without any clear reason or warning. Full definition
Did the iOS 10.2.1 update help in reducing random shutdown issues on your iPhone 6 or iPhone 6s? (iphonehacks.com)
Interestingly enough, Cook says that its initial effort to stop random shutdowns of older iPhones did the job it was meant to, but it was not clear at the time (the feature was launched last year) that it would make some older iPhones slow down. (iphonehacks.com)
This leads to them degrading faster than usual and cause random shutdowns of the device. (iphonehacks.com)