Welcome to Android Central! It could be sure to some app you installed recently--can you think of any that might have control over the screen?
Another possibility is that it's being intermittently exposed to a magnet that is activating the phone's smart cover sensor. A magnet in the right place will make the screen turn off, and then taking the magnet away will awaken the screen.
Posted via Android Central App--please excuse the brevity and any typos!