I watched it a few times and it seems like after the Japanese landed that jolting left Jab he stepped on the front foot of the South Paw, accidentally of course, and there's a kind of delay of the right hand because I think it looks like he had only been thinking about throwing the Jab but Payano appear to trip backwards in an odd way and it was then that the Japanese saw him off balance and saw the opening and followed with a sort of delayed straight right hand knockout

Right after the jab lands you can see it