From my own experience...
I pushed my green horse to far, to fast. I asked her to flex to the left side and held steady pressure on the rein, I recall the typical warning sign as pinned ears, that's it. She immediately went up and over with me. I shook the dirt off and asked myself "what did I do to cause this" I stood there for a second, checked her out and got right back on. I changed my tactic and within 10 minutes I had her flexing perfectly to both sides.
She did it because of me, she was confused and frustrated. I put no blame on the horse... Luckily she never flipped again, I took everything slow with her from then on.
Some horses react differently from others, I do wonder if the horse in question has had a vet check thought?