If your tendency is to stand up through the impact area, you’ll tend to release it early and hit a lot of thin shots. Most likely you have lot of tightness in all the muscles in the back of your body (erector spinae, gluteus maximus, hamstrings, gastrocnemius) and this tightness doesn’t allow you to keep the forward bend in your golf swing through impact.
Another reason people stand up through impact is because they release the club early on downswing. If they kept their spine angle correct, while throwing clubhead at the ball, then they would stab the clubhead in ground. So they instinctually stand up to give room for clubhead to sweep the ground. These people tend to hit their tee ball better than they do their irons because the ball is on a tee for a more sweeping motion.
So it is important to address the physical issue as well as the swing mechanical issue.
In my Cardiogolf program, I like to alternate between exercises that work the muscles used in the golf swing and drills that help you feel the correct motion. So here is a stretch to help you loosen up the muscles in the back of your body and a swing drill to help you feel how to stay down and through the impact area.
Forward Bend Stretch
Forward Bend Step Drill