What's the cause of this Leica M6 light leak?

At first glance, I suspected a bad seal on your camera back. But then I noticed the pattern on the last sample image. It looks like the light leak occurred during development, due to the light "shadow" (for lack of a better term) of the sprocket holes along the bottom edge. The light had to leak onto the film when the film was wound loosely. Are you developing your own film, by chance? Your development tank would be my first place to check if you are.

Considering the shadows of the holes on the last one and how they get much less noticeable, it has to be a hole in the side of your developing tank.

No, this does NOT look like a light leak during development.

This is most likely a light leak at the top of takeup area in the camera. Most cameras move the film from the cartridge at left to the takeup spool at right, when looking at the back of the camera and the lens pointed away from you normally. Since the scene is projected onto the film plane flipped about its center, light showing up at the bottom of pictures is actually hitting the film at the top. The light leak is therefore at the top right corner of your camera.

The light leak can't be in the middle since there is no way another layer of film could cast the shadow seen in your last frame.

The leak isn't in the cartridge since it appears in the same place in every frame. There isn't exactly one frame per winding in the cartridge. Even if there was at one diameter, this wouldn't be true for higher or lower layers. These would be at a different diameter, and therefore a different frame stride per revolution.

The reason you see this in some frames than in others is that it depends on how long the frame sat in the right spot on the takeup reel. In other words, how long you waited after winding the film before winding it after taking the next frame. If you took several frames in rapid succession, you may not see a leak except on the last one. Your bottom frame spent a long time in the second to last position on the takeup reel. The frame right after it should should the effect quite seriously.

It should be obvious this is not a developing problem for two reasons. First, there is no way to explain how the leak is in the same place in each frame. Second, developing tanks hold the film by the edges. You would see a shadow of the grooves or spirals of the tank if the film was being exposed in the tank.