I'm curious, because I really want to know how this works. Is that to raise or lower the pressure in the case - which will raise or lower the velocity?
I think what happened in your case is your throat eroded and your bullet is jumping more. You might have had a marginal load to begin with and that little change in jump might have ruined it.
When I find a node I map it out by powder charge vs OAL. I find the middle of the oal range and the middle of the powder charge range and use the load where the two intersect.