I can't find details about the sensor read out on the 5D, but the only way this could be a shutter problem is if the sensor read out is top to bottom then left to right (which is not the way it is generally done on newer models, but I'm not sure how they may have done it on older models.) Even then, it would seem a bit odd that it spends so long with the bottom of the image blocked. If the shutter was lagging, I would expect it to continue to lessen the amount of black at the end.

My best guess (though it is still a guess) is that this is not shutter related. At least not in the traditional shutter lag way.

I can't find details about the sensor read out on the 5D, but the only way this could be a shutter problem is if the sensor read out is top to bottom then left to right (which is not the way it is generally done on newer models, but I'm not sure how they may have done it on older models.) Even then, it would seem a bit odd that it spends so long with the bottom of the image blocked. If the shutter was lagging, I would expect it to continue to lessen the amount of black at the end.

My best guess (though it is still a guess) is that this is not shutter related. At least not in the traditional shutter lag way. A more drastic shutter failure could still be to blame potentially, but it would be an unusual mode of failure.

Best bet would probably be to see if you can send the images to Canon and get feedback from their repair people. They would be far more knowledgeable about the rarer failure modes.