If you have GPS enabled, or even if you keep Wifi enabled while you are away from home (as myself and most people do)- there is a surge in power demand (esp the wifi antenna). Have you tried, as an experiment, go outside with wifi bluetooth gps turned off? Personally, my GPS is always off as it a significant power drain hog. And without an analysis of you iPhone, I mean oscilloscope, ampermetre and a few other tools in a lab, cannot speculate as to what exactly causes it other than a process of elimination. With special tools one can measure exactly the amperage drain, if the voltage is steady, if the phone self shorts. As users, through support, the best initial method is factory reset/reinstall, and so on after that.
As for what did the update trigger, several ways of looking at this. Even if I had a very good idea, would not want the post assuredly deleted. Also, engineers are working on it already, including 3d parties no doubt. Maybe the issue is already known but whatever happens next is Apple's call.
"the unexpected shut down should always happen, shouldn't it" There is no clear common shutdown denominator here. But let's assume that it is hardware, the answer is simpler. Automatic shutdown are generally demand and drain relative. A worn out Li Ion will shut down frequently under demand (amperage spikes) but not under steady states or very low discharge. Add to it built-in sensor related circuit breaker safety shutdowns, there are many ways it can happen. Would not worry too much about the gritty scenarios, other than the most tangible, best solution for you.
