How would you explain what happened?
Doesn't know how to use the brake properly, and mashed the brake on purpose or not. Even if he slid on the expansion joint, personally I know what it takes to flip a bike over doing a stoppie and it takes a lot of either leaning forward over the front of the bike or just way too much brake. At the speed he was going he would have had to use a stupid amount of sudden brake to flip the bike over like that.