This is less likely, but make sure there aren't any strong magnets near the phone. Devices that use magnetic sensors to activate a Smartcover function (i.e., turning off the screen when closing a smartcover) might react that way if there's a strong magnet right near the phone.
Hmm. When you say it's a magnetic shutter, do you mean the cover stays shut thanks to a magnet, or do you mean that it's a smartcover that automatically turns off the screen when the cover is closed thanks to a magnet embedded in it? In either case, take the case off and see if the problem still occurs.