Definitely weird.
I imagine you still have your "bar tan", too.
So far the people who have mentioned experiencing this are older guys.
Wonder if any of any of the SSC's older trainees also have it. If it occurs with any frequency in older populations, might be a nice thing to know about, yeah?
I had a similar situation, in which the bar ended up tearing open the skin on my back. I was not using a bar with any sort of sharp knurling, it was not rolling around on me during the squat, and once I noticed the bar was irritating my skin I doubled up on T-shirts, but it didn't work. At first it was just an irritation and some amount of irregular markings on my back, but eventually it turned into large open sores that soon got infected. This happened when I first switched from High Bar to Low Bar, and I never had any problem with my skin while high bar squatting. It was definitely weird and took some working around.
I got a couple of neoprene sheets (http://www.foambymail.com/NE/neoprene-foam-sheets.html), cut them down to size, and taped them over the area around the spine of my scapula. Basically it was a pussy pad that I could (sorta) hide under my t-shirts. My ego couldn't handle a full blown bar pad, and I work at a university. If any of my athletes saw me squatting with a pussy pad, all credibility I ever had would be tossed out the window. I wasn't going to stop my linear progression, so I just squatted with the pads and tried to let my wounds heal in between squat workouts. As they healed, I decreased the thickness of the padding from week to week until eventually the open sores had closed and become somewhat of a calloused area of skin. Throughout the process, my skin seemed to adapt and I don't have that problem anymore.
A couple things to note. I sent a picture of my cuts to Rip and he told me to move the bar down a little and see if that helped with the open sores getting aggravated (the bar may have been sitting directly on the spine of the scapula and causing the rips). It helped. Also, squatting with the pads was not ideal. Bar placement was tough in terms of grooving in where the bar should sit. It also feels really wimpy to squat with any padding on your back. Its like drinking whiskey with diet coke.
I know of a case where hormonal changes (related to aging) caused allergic reactions (to food and other substances). It went away over time (it took several years if I remember right). I don't know if it is common.
One other thing to consider is chalk. In these cases, was there a lot of chalk on the back of a t-shirt and the bar is working it into the skin? Seems like a harder area to scrub clean. I checked google for chalk allergies and didn't find much aside from people reacting to the casein in dustless chalk, but, anyway, just one more clue.
As for changes in aging lifters, I've been under all kinds of bars from cheap and dirty to good, since the early 70s, and have not had this reaction. Hope I don't.