A simpler way to look at this is to ignore the exact chip formation mechanics and simply look at planing forces. As a few people have noted in this thread, tightening the cap-iron set increases planing effort/force. That increased force must be acting at or just above the point of the cut (since the rest of the plane<->wood interface hasn't changed), and will logically produce increased blade wear.
Brian's mechanical explanation makes sense, though I'm not 100% convinced that's the only thing at play. Wood is a complex material, and we're talking about stuff that occurs on relatively small scales.
EDIT: Steve Elliott has posted pictures from the Kato/Kawai study
here, showing wear on the back of the blade just above the cutting edge, where the shaving turns as Brian describes, so I think it's safe to say that that wear mode is indeed a contributor :-)