The bug was that it no longer shot where I was pointing the mouse unless I let go of shooting for a second. I noticed that it happened right when I bought the 3rd-5th "spread".
The turrets also started doing it too; where they constantly shot in the same direction with no more tracking, after destroying a robot, but then they reset to a new target direction after they stopped shooting. So if a robot walked into the path, they'd continue shooting in the same direction.