Stump,
Stuff like this is why I have 3 sheets of plywood behind my target. (3) 4x8x3/8 sheets works well for a back stop. When I first got back into bows I was kinda erratic...okay I couldn't shoot for crap and didnt wanna kill anybody in the neighborhood.
A gust of wind(crossing) between you and the target could cause this too.
HWD