A right handed shooter will shoot left when tired. You're muscles will tire, sometimes not until you are at full draw and aiming, and you will overcompensate for the tired holding arm and push a little harder with it to hold it steady, in effect, pushing the shot left, or dropping it. It'll show up as a low hit, or left hit. If yourexperiencing this after 15 shots @ 70lbs., you need to drop down quite a bit.

Mikey