I had heard it mentioned a few times how David had tripped as he was trying to execute that single on Yazdani. I originally thought that also, but if you have the ability to re-watch the video (5:36 in the subscriber-only video on Flo) you can see how as David has his left leg off the mat in an attempt to trip Yazdani, Yazdani takes his right hand to the right side of David's face/head and forcefully pulls David to David's left, causing him to lose his balance since at that moment he had his left foot off the mat. It was great defense by Yazdani and very well timed. It was also a critical moment in the match. I am always amazed how many things I miss when watching the action live.