I noticed the same thing was happening to me this year.  I finally read an article about how pro golfers eat throughout their rounds, and started keeping almonds or peanuts in my bag, and taking a banana along.  I try to have protein before I tee off, whether an egg or some turkey, and then every few holes, munch on some almonds, and eat the banana at the turn.  Something to do with energy and blood sugar. 

Other times, I was just losing focus, or getting ahead of myself (getting in my own head) because I thought I had the round of my life going.

My advise is to forget about how you've been playing so far and concentrate on playing the next hole. My best score this year was 69 and when I got on the 10th I was a little nervous, but when I focused on making a par, I did.

Agree sometimes the turn is costly. Think fatigue, or thinking too much. I never look at score card longer than it takes me to put a number in the current box. Try really hard to stay out of my own way!  I'll grab a protein bar and more water for the back. Even though it's getting cooler. still need to stay hydrated!  Good luck!