I'd make a silly reckless guess and say that there are probably seals in the stalk slides to keep the water in until the stalk is fully extended - and it is them that have failed, nice investigation job for when you get it off, I thought, but never needed to try on my old Passat B5, that it was possible to unbolt the washer from through the hole in the bumper - a lot easier probably to get the bumper off though, just needs the confidence to get round to doing it - I'll need to muster that inner "strength" when I get round to attempting a front parking sensor retro-fit!
Edit:- if/when you pull the stalk fully out, the washer fluid path will be open, so if there is any residual pressure in there it will dribble?