Nothing peculiar about it unless you've never shot a rifle off-hand before.
The first shot was his best shot because every shot after that was under stress. He was using an unsupported AR15 with a Red Dot Sight in the (presumably) prone position. RDS is most likely unmagnified and has a 2-3 MOA reticle. That, combined with shooting unsupported, and being under stress = "spray and pray" after the first shot. A small change in POA = a fairly large change in POI. This should be immediately evident to anybody who shoots and understands angular measurements. Heart rate goes up, breathing quickens and shortens, adrenaline pumps... all of that means that RDS was probably dancing all over the place without shooting supported.