The phone has like 70 processes running all the time. If one of them does something to get it's attention, the screen will wake up. One of the times it happens to me is when using adb and typing "su". Even though nothing on the screen moves around or changes and I haven't touched the screen or any other part of the phone, it does light up.
You may have some program running in the background sending/receiving data from the internet. Or you could have one that periodically runs something with su.