Best probability is that the shooter inadvertently squeezed trigger in his initial positioning of the rifle atop the firing platform of boxes.
So that means 1st shot not fired until at least Z160-170 where it can be accomplished without shooter standing up.
at that angle at least, there’s a better probability the bullet could have ricochet off the pavement.
Otherwise if not striking the pavement, instead striking grass/dirt then there’s the manhole cover to consider and why that one FBI agent was bending down to touch that uprooted grass by the manhole cover with his fingers.