Quick answer: Face-on: at bunker address the weight is split evenly or back, and during the shot the body leans away from the target trying to lift the ball over the lip.
What it does to the ball
The low point moves back: the club enters the sand too early and the ball stays in, or catches it clean and rockets it.
Root causes (by frequency)
Trying to help the ball over the lip by leaning backwards
Copying the full-swing setup into a shot that asks for another
How to fix it
Anchor the weight on the lead foot (60-70%) from address and keep it there for the whole shot: the club will enter the sand in the same spot every time
Drills that fix it
Anchor the lead foot · 10 min · 15 reps
Set the weight forward for the entire bunker shot Dig your feet well into the sand and load 60-70% of the weight on the lead foot at address. Hit bunker shots feeling that foot as an anchor that never moves: if the body leans back to lift the ball, you'll feel it instantly. The height comes from the loft and the sand, not from your lean.
Frequently asked questions
What is "Weight back in the bunker" in the golf swing?
Face-on: at bunker address the weight is split evenly or back, and during the shot the body leans away from the target trying to lift the ball over the lip.
What does weight back in the bunker do to the ball?
The low point moves back: the club enters the sand too early and the ball stays in, or catches it clean and rockets it.
How do you fix weight back in the bunker?
Anchor the weight on the lead foot (60-70%) from address and keep it there for the whole shot: the club will enter the sand in the same spot every time
Sources
Stan Utley (con Matthew Rudy) — The Art of the Short Game