That is one of the great challenges of golf. I've done it a hundred times. It's called choking. There was a book written 30 years ago called "The Inner Game of Golf". It talks about comfort zones. You weren't comfortable being -1 after 12. It wasn't your norm. So you subconsciously reverted back to your comfort zone. Of course we have the law of averages in play too. But when you're playing well, exceptionally well compared to your norm, you get uncomfortable and let your mind become overly analytical in the physical process of hitting shots.
Any time I'm playing better than usual ( for me it's when I get 3-4 under at any point in a round these days) I get nervous. I freely admit it. I shouldn't. I've shot hundreds of rounds in the 60s in my life. But I get nervous and lately I have trouble finishing a round. When I'm even par after 15 or even -1, I'm fine. But when I have a chance to shoot in the 60s, I tend to gag these days. The key is amnesia. Forget about what you've done and where you are and concentrate on the next shot. Easier said than done.
My son told me a story a couple summers ago after he came him after playing a round with Scott Stallings. He said they were somewhere on the back nine and Scott had reeled off a bunch of birdies. My son asked him how many under he was. Scott's reply....I have no idea. He was completely comfortable.
Nice round anyway though!