What I was doing: Standing Barbell Row with #255 (the very first rep.)
How it happened: I rolled my back forward at the top of the pull.
What I could have done to prevent it: Not break form, dial back the weight for higher reps, warm-up better (drilling form.)
My head wasn't really in the game this morning. I did a few sets of Pull-Ups, a few sets of seated cable row, and then racked up #255 and went to do a standing row. After the first one, I just set the bar back down and called it a day. I knew I had screwed up, and that it's my own fault for being a moron.
I had it rolled out (OMG that is uncomfortable.) I'm going to finish out the week without performing anything that places stress on the lower back, and reanalyze the discomfort later. If all seems well, I'll be doing some light PT of a weightless reverse hyperextension. It seems to be located around the quadratus lumborum, but it's hard to be certain. There's a lot of stuff in that general location.
Note: I (of all people) should know better. Learn from my mistakes, and don't break the lordotic curve under load.