anecdotal counterpoint :) -- squatting aggrevates my lower back. I think its because my hamstring flexibility is terrible which causes my squat form to be terrible. I'm working on that.
I also have flexibility issues (I'm tall), and I used to strain my lower back on heavy squats until I widened my stance and pointed my toes a bit more outwards, to prevent my lower back from rounding. You could ask someone to check if your lower back arches towards of the downwards movement.
