The most common cause of a bad back is keeping your back foot flat on the ground in the golf swing; get up on your back toe to eliminate excessive torque
Possibly the worst advice anyone can get is to keep their head down during the swing, this completely restricts the turn, you want to keep your head centered not down, so that you can make a centered pivot with no sway or spine angle break down
Many slicers set up with their right shoulder too high, prior to bending over to get in to your stance try tilting your right shoulder slightly down then take your stance, this will allow your body to work more around your right side and eliminate that steep downswing