Re: HTC One X Airplane Mode
Welcome to Android Central! If the phone is starting to overheat, it might be a safety mechanism for the phone to turn off the cell radio. Phones often have a Cooldown mode that turns off functions like cell radio temporarily. Overheating can occur sometimes if cell signal is poor, because the radio starts to work overtime to search for signal. Was it weak at the time?
Another possibility is some app misbehaving, and perhaps excessively accessing the web. Try installing [Only registered and activated users can see links. Click Here To Register...] to get a breakdown on what's using battery and CPU.