Definitely a light leak. If you only get this near the beginning of the roll, then I'd go with Gary's theory--don't load the camera or leave film in direct sunlight.

If it happens in various places on the roll, then I suspect it's time to send your camera to have the light seals replaced (usually a good time for a general CLA as well--the mirror damper is often made of the same material as the light seals). Light leaks like this will vary from frame to frame, because sometimes the leaky side of the camera isn't exposed to direct light, and leaks are more likely to show up when the film isn't advanced for a long time.