Because of his relatively short range and lumbering movement, Reinhardt rarely threatens from the high ground. Neither does he often pull off sneak attacks that take the other team by surprise.
The clip begins with NoNovacaine and their team preparing to leave spawn at the start of a King's Row attack. Their Mei has used Ice Wall to boost NoNovacaine's Reinhardt all the way to the top platform, where snipers typically enjoy positioning. NoNovacaine hides out of sight, keeping their presence secret from the enemies.
As soon as the countdown hits zero, beginning the assault, NoNovacaine rockets out of the top door with a charge, aiming straight for the main gate to the point, hoping blindly to strike some enemy and pin them to the wall. And because no one expects a flying Reinhardt, their mission proves successful: Some poor Mei, living in the world of reason, has no way to anticipate all 7-feet-4-inches of Reinhardt barreling toward them from on high, and the Overwatch veteran crushes her against the statue of Mondatta for the first kill of the game.