I don't know why...
As others have stated, a reliable, repeatable pre-shot routine is a good thing. It can provide the confidence that conditions are right for a successful shot. Most aiming, aligning will occur while standing. Getting down on the chosen shot line, bridge placement, bridge length, body and feet alignment and spacing...all the repeatable checklist items in preparation for striking the cb.
Ideally, each successive warm up stroke, further confirms/slightly adjusts the perceived proper shot and stroke line as well as the speed and english to be used. And then...
Then there is the dreaded 'final stroke'

...when all that preparation can be derailed. Tension in my stroking shoulder can pull/push the delivery off line on the final stroke.
Something that helps me, when this is going on with me is to practice with some changes...If you have a pause in the final stroke, at the cb, or especially at the end of the backswing...try not stopping. Let one of the last 2 or 3 continuous warm up strokes be the delivery stroke. For me, sometimes the stopping creates tension that results in off line delivery.
Sometimes, in practice, as the final stroke is delivered, I close my eyes at contact and then peek at the results after hitting the cb.
For me, I'm trying to remove tension from the shot...physically and mentally.
I try to lie to myself. I want the shot process, including hitting the cb to be part of an overall equally importance-weighted continuum...not a build up to 'bam'.
'Ready on the left! Ready on the right! The range is clear....FIRE!!!'
..sometimes creates shoulder tension for me, when a clear distinction is made between preparations and execution.
Like I said, I don't know what the source of the problem is that OP is experiencing, or how to overcome it. But, I think most of us can relate to the issue.
But for me, I change things in practice, and Lie to myself. If that doesn't work..beer or Robitusson, but not both.
