It's a combination of many things. Wifi activity can heat the device and the CPU is doing practically nothing.
So Sync with Onedrive can heat your device.
Chrome and hangouts will heat your device but decreasing the video bandwidth will help a little.
So far in my short-term experience with my SP3 this makes sense. I've noticed the fan spinning up fast when downloading Photoshop and Lightroom and installing them, and when syncing with Dropbox.... the only other time was running LR and skipping quickly through RAW previews in the library view.. I have the LR catalogue placed on OneDrive though... so actually that was possibly to do with syncing to the cloud as well come to think of it...!
When I'm just surfing on IE (either desktop or metro as so far I've not got my head round which I'm using...lol) I haven't noticed the fan - it's been good as gold and very cool. I'm seriously impressed with it... this is a i7 shoehorned in a tiny space...!!