Sounds like your squat form might be off. I think the bar should be over the mid foot on the low back squat so going backwards should not be likely.
If you look at Rippetoe's squat vids, he says that the eyes should be gazing at the floor ahead, and that the torso is forward, not upright. Also, from the bottom position, the point of focus as you come up is the lower back around the sacral area.
The bar should never feel like it is dragging you back.
Also, drop the weight, work on form, then progress