Basing on reports from Geekbench people noticed that their iPhones begin to work slower over time. Smartphones worked in slower way and many users had problem with unexpected shutting down of their devices. Apple solved this problem in iOS update to 10.2.1 version, but they haven't said what was the reason of this bug. Today Apple published official statement in which we can read that company slowed down their devices on purpose, and that's why phone didn't reset.
When our phone needs a lot of power to work there's a chance that phone will reset because when battery is on low level it's cold, and then in some cases battery may not transfer enough amount of power. To keep our phones working in fast way there's only one solution: replace of the old battery with reduced capacity by high voltage in the phone to the new one. How we can read in official Apple's statement this problem is connected only with iPhone 6, iPhone 6s and iPhone SE, but with update of iOS to 11.2 version Apple also reduced speed of iPhone 7.