One word...Fatigue. When you are physically fatigued is when bad habits start, whether its hips, tempo, etc. Stop at the turn take a quick break, grab a bite to eat or bring a banana or get a light sandwich in the clubhouse and a drink. This helps keep you energized for the second nine and helps you relax. The other thing that I have found is that on the back nine players tend to get into their own head and try to figure out what they could shoot for the round on hole 12. Take it one hole at a time to stay strong mentally, and a snack or two to stay strong physically. I do this and my back nines are generally lower than my front nines.