Forum user Eccanz describes the problem well over on the Apple support forum:
Woke up this morning and my iPhone started to restart by itself. I can use it to make calls without issue, but apps and text messages have to be quick Before it quickly restarts to a black screen and the spinning star thing before it is fine for another minute.
In the run-up to the crash, handsets are reported to heat up noticeably.
The problem it seems is a bug in iOS 11 that causes processor usage to spike to 100 percent when apps generate a notification. This causes the device to heat up, which, in turn, causes a reset.