My iPhone 5S is heating up occasionally. So I was trying to figure out what is the reason. Of course there are obvious reasons such as revving up the CPU with too many applications running in the background but even after killing all those apps my iPhone 5S continued to heat up in specific situations which I could not find a reason for the heat up.
I figured out the reason, well in my case at least. The iPhone will try its best to connect to a good 4G network all the time and so if you are in an area where there is no good 4G network (these bad signal pockets are all over if you are on AT&T, specially indoors or in remote areas or when you are driving and using the 4G service such as running GPS and a map application) then your iPhone will definitely heat up. To prove that, just switch on the airplane mode and your iPhone will cool down immediately.