Nice shooting, Lead Pipe... Were you 'covering' the X with the front dot, or were you putting the X just above the Front sight (pumpkin on a post)?

If you were covering the X, it looks like you started anticipating the recoil and twisting downward. I'm guessing that the first few were dead-on and the low shots were after anticipation set in.

If you were using pumpkin on a post aim, start covering the X.