Why are my devices shutting down for no reason?

Hi All, long time follower, first time questioner... I really do have to say a quick thanks to the community before I start!

Anyway, I repair phones for a living. I've had quite a few 5S and 6S plus iPhones turn off for no reason. Actually quite a few iPhones. Even after replacing the battery. Which kind of makes it more annoying.

Anyone who has seen this trend before and knows why it happens, that'd be really helpful. The devices do charge just fine. Yes, I cycle through the batteries.

Thank you