That has happened to me a few times. I just assumed that I had ‘fat finger syndrome’.
It is VERY annoying when it happens but so far I haven’t broken anything.
In my case it’s the mouse button but I do not think it is sticky since it only seems to do this for Z.
Now there IS a chance that if I press the mouse button in quick succession (ie small step and need to move down a ways) that gSender thinks the button has been pressed for a longer period and decides that a continuous move is requested. This might happen if gSender just looks at the button status irrespective of there being more mouse clicks (press mouse button, release mouse button, press mouse button again, gSender checks if mouse button is active and due to quick clicking it happens to be depressed - continuous move is called for). I do not know enough about windoze programming to know if this is a realistic situation or not but it sure explains things.
X and Y motion usually doesn’t involve mouse clicks in quick succession so the issue doesn’t happen with those axis.
Debouncing might also come into play …
Oh, I should add that in my case the downward motion happens for a limited distance (I would never be able to react fast enough if that wasn’t the case)