I don't know if what Hudson does is necessarily a balk as long as...
He does it the same way every time.
He comes to a discernable stop the second time. I guess the umps are interpreting his first stop as still being part of his "Stretch" move.
I just learned the balk rule last year and when I went to opening day I noticed that the pros do many things that are technically illegal...
Pitch off the end of the rubber.
Intentionally walk a man with the catcher starting out outside of his box.
The bottom line is that a balk is what the ump says is a balk (which is why it helps to discuss exactly what parts of the balk rule will be called before the game).
In my experience, many of them only focus on things that fall in the realm of trying to deceive the runner (and are more loose when it comes to definition of what is a legal delivery).