Oh, thanks for the help, but I already knew how to stop it. I usually squat pretty narrow. I widened my stance, as per Rippetoe, and pushed my knees out more. The wider stance stopped me from sinking as deep. I still wonder if I should do it a bit more narrow, though. If that tucking of the pelvis isn't harmful, I'd rather do that. I like the feel of sinking that much, as opposed to going wider as Rip recommends.
I'd love to hear from some more experienced squatters on this (i.e., MC, Glenn, CCJ). Too bad I can't get a video up. I'd just squat with the bar to show both forms I use when squatting, and let someone with a lot of knowledge critique me.
Thanks for the help though, man.