Shots spiralling to targetl

[alwie]

I loaded up some Hornady 105 gr BTHP with some N540 using Starline brass. And I was spotting for my son while he was walking in on a target at about 500 yards out.  As I was watching it seemed like some of the rounds were “spiralling in” (following a helical path) to the target while others were following a standard parabolic curve.  From a reloading perspective, what did I do wrong to the rounds that were spiralling in?

Thanks