What are these diagonal marks on my innertube? I'm on my second inner-tube in as many weeks and the culprit seems to be the same between them; these diagonal marks on the side around the whole of it&mdash;but only on one side (sorry, didn't make a note of what side that was when removing it :().  Some are shallower, some deeper; the second picture below is the one that was actually leaking.

When I took it in to my LBS the guy said I pinched the tube with the tire bead when installing it, but I have a hard time imagining that I somehow managed to pinch the entire tube all along one side, in such a regular pattern (the marks repeat regularly, every 1"), twice in a row.  

I wouldn't call myself a bike expert, but I have, from time-to-time, replaced tubes; did my front wheel and it's been fine for months.  Did I just get lazy/cavalier in my tube installs or is there some other explanation?
Those are scuff marks from running the tire flat (or nearly so), and/or from a tire whose sidewall is breaking down.  

One suspects that underinflation played a significant role in this tire failure. If you can see "ripples" in the side of the tire while you ride it, it's way under-inflated.  One wall of the tire will tend to fail before the other, based on the rotation and the diagonal cord alignment. The spacing corresponds to where the tire rippled as you rode it -- it repeats in the same location on each rotation since the sidewall gets softer and softer there.