Any sort of heavy use can create heat (data, CPU, etc) ... some due to the various wireless radios and some heat due to the heavier battery drain. I have noticed the 4G radio can make it run a bit more hot in general even without heavy traffic as well. So if everything is turned on (GPS, 4G, WiFi, Bluetooth) I'm sure that can cause a higher average temp, not counting any background apps that may be doing frequent data syncs.
As far as your temps though, I wouldn't worry about it. At home (72-75 air temp) mine ranges from 80 to 100 depending on what I'm doing, typically with GPS and WiFi radios on, 4G/Bluetooth off.
Until you pass up 115+ I don't think there is much of an issue. It has been 100+ degrees outside (110-115 heat index) where I live for the past week, and my phone pushes past 125 degrees while I'm in the car (A/C is broken and dark interior, so my car temp is always higher than outside air even with windows open). After about 120 Battery Indicator (3rd party app) shows "overheat" and the phone stops trying to charge, I have just been shutting it off after that. Hopefully after today the car temps shouldn't be as much of an issue for a while since the high temps should be no more than 90 next week, but I'll still be paying attention to see if it peaks past 120 if I'm running GPS or heavy data while it is in the car.